Q:

/usr/bin/ld: cannot find -lX11

https://github.com/laazyboy13/linux-csgo-external/issues/6
How do I get it to find libx11-dev?
william@william-elementarylinux:~/csgohack$ make
Linking CXX executable build/csgo_external
/usr/bin/ld: cannot find -lX11
collect2: error: ld returned 1 exit status
make[2]: *** [build/csgo_external] Error 1
make[1]: *** [CMakeFiles/csgo_external.dir/all] Error 2
make: *** [all] Error 2
william@william-elementarylinux:~/csgohack$ ld -lX11 --verbose
GNU ld (GNU Binutils for Ubuntu) 2.24
  Supported emulations:
   elf_x86_64
   elf32_x86_64
   elf_i386
   i386linux
   elf_l1om
   elf_k1om
   i386pep
   i386pe
using internal linker script:
==================================================
/* Script for -z combreloc: combine and sort reloc sections */
OUTPUT_FORMAT("elf64-x86-64", "elf64-x86-64",
          "elf64-x86-64")
OUTPUT_ARCH(i386:x86-64)
ENTRY(_start)
SEARCH_DIR("/usr/x86_64-linux-gnu/lib64"); SEARCH_DIR("=/usr/local/lib/x86_64-linux-gnu"); SEARCH_DIR("=/usr/local/lib64"); SEARCH_DIR("=/lib/x86_64-linux-gnu"); SEARCH_DIR("=/lib64"); SEARCH_DIR("=/usr/lib/x86_64-linux-gnu"); SEARCH_DIR("=/usr/lib64"); SEARCH_DIR("=/usr/local/lib"); SEARCH_DIR("=/lib"); SEARCH_DIR("=/usr/lib");
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==================================================
attempt to open /usr/x86_64-linux-gnu/lib64/libX11.so failed
attempt to open /usr/x86_64-linux-gnu/lib64/libX11.a failed
attempt to open //usr/local/lib/x86_64-linux-gnu/libX11.so failed
attempt to open //usr/local/lib/x86_64-linux-gnu/libX11.a failed
attempt to open //usr/local/lib64/libX11.so failed
attempt to open //usr/local/lib64/libX11.a failed
attempt to open //lib/x86_64-linux-gnu/libX11.so failed
attempt to open //lib/x86_64-linux-gnu/libX11.a failed
attempt to open //lib64/libX11.so failed
attempt to open //lib64/libX11.a failed
attempt to open //usr/lib/x86_64-linux-gnu/libX11.so succeeded
-lX11 (//usr/lib/x86_64-linux-gnu/libX11.so)
libxcb.so.1 needed by //usr/lib/x86_64-linux-gnu/libX11.so
found libxcb.so.1 at //usr/lib/x86_64-linux-gnu/libxcb.so.1
libdl.so.2 needed by //usr/lib/x86_64-linux-gnu/libX11.so
found libdl.so.2 at //lib/x86_64-linux-gnu/libdl.so.2
libc.so.6 needed by //usr/lib/x86_64-linux-gnu/libX11.so
found libc.so.6 at //lib/x86_64-linux-gnu/libc.so.6
libXau.so.6 needed by //usr/lib/x86_64-linux-gnu/libxcb.so.1
found libXau.so.6 at //usr/lib/x86_64-linux-gnu/libXau.so.6
libXdmcp.so.6 needed by //usr/lib/x86_64-linux-gnu/libxcb.so.1
found libXdmcp.so.6 at //usr/lib/x86_64-linux-gnu/libXdmcp.so.6
ld-linux-x86-64.so.2 needed by //lib/x86_64-linux-gnu/libdl.so.2
found ld-linux-x86-64.so.2 at //lib/x86_64-linux-gnu/ld-linux-x86-64.so.2
ld: warning: cannot find entry symbol _start; not setting start address

EDIT: 
I had a look at https://github.com/laazyboy13/linux-csgo-external/issues/5 and new error:
william@william-elementarylinux:~/csgohack$ sudo apt-get install libx11-dev:i386 libx11-dev 
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following packages were automatically installed and are no longer required:
  libdrm-dev libexpat1-dev libfontconfig1-dev libfreetype6-dev libice-dev
  libjbig-dev libjpeg-dev libjpeg-turbo8-dev libjpeg8-dev liblzma-dev
  libpng12-dev libsm-dev libtiff5-dev libtiffxx5 libvpx-dev libxcb-dri2-0-dev
  libxcb-dri3-dev libxcb-glx0-dev libxcb-present-dev libxcb-randr0-dev
  libxcb-render0-dev libxcb-shape0-dev libxcb-sync-dev libxcb-xfixes0-dev
  libxshmfence-dev x11proto-damage-dev x11proto-dri2-dev x11proto-fixes-dev
  x11proto-gl-dev x11proto-xext-dev x11proto-xf86vidmode-dev
Use 'apt-get autoremove' to remove them.
The following NEW packages will be installed:
  libx11-dev libx11-dev:i386
0 upgraded, 2 newly installed, 0 to remove and 4 not upgraded.
Need to get 0 B/1,256 kB of archives.
After this operation, 5,020 kB of additional disk space will be used.
Selecting previously unselected package libx11-dev:amd64.
(Reading database ... 215279 files and directories currently installed.)
Preparing to unpack .../libx11-dev_2%3a1.6.2-1ubuntu2_amd64.deb ...
Unpacking libx11-dev:amd64 (2:1.6.2-1ubuntu2) ...
Selecting previously unselected package libx11-dev:i386.
Preparing to unpack .../libx11-dev_2%3a1.6.2-1ubuntu2_i386.deb ...
Unpacking libx11-dev:i386 (2:1.6.2-1ubuntu2) ...
Setting up libx11-dev:amd64 (2:1.6.2-1ubuntu2) ...
Setting up libx11-dev:i386 (2:1.6.2-1ubuntu2) ...
william@william-elementarylinux:~/csgohack$ make
Linking CXX executable build/csgo_external
CMakeFiles/csgo_external.dir/main.cpp.o: In function `main':
main.cpp:(.text+0x337): undefined reference to `XOpenDisplay'
main.cpp:(.text+0x37f): undefined reference to `XKeysymToKeycode'
main.cpp:(.text+0x3bb): undefined reference to `XGrabKey'
main.cpp:(.text+0x3d4): undefined reference to `XSelectInput'
main.cpp:(.text+0xc7a): undefined reference to `XNextEvent'
main.cpp:(.text+0xcca): undefined reference to `XUngrabKey'
main.cpp:(.text+0xd03): undefined reference to `XGrabKey'
main.cpp:(.text+0xd1c): undefined reference to `XSelectInput'
main.cpp:(.text+0xd2d): undefined reference to `XPending'
collect2: error: ld returned 1 exit status
make[2]: *** [build/csgo_external] Error 1
make[1]: *** [CMakeFiles/csgo_external.dir/all] Error 2
make: *** [all] Error 2

EDIT2:
$ apt-cache policy libx11-dev
libx11-dev:
  Installed: 2:1.6.2-1ubuntu2
  Candidate: 2:1.6.2-1ubuntu2
  Version table:
 *** 2:1.6.2-1ubuntu2 0
        500 http://nz.archive.ubuntu.com/ubuntu/ trusty/main amd64 Packages
        100 /var/lib/dpkg/status

$ strings /usr/lib/x86_64-linux-gnu/libX11.so | grep -P '^XOpenDisplay'
XOpenDisplay

EDIT3: I followed @xiaodongjie's answer's steps and the CMakeLists.txt file. I used github.com/laazyboy13/linux-csgo-external from the 15th commit. I edited the linux-csgo-external-master/CMakeLists.txt file to 
cmake_minimum_required(VERSION 2.8.4)
project(csgo_external)

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -m32 -std=c++11")
set(EXECUTABLE_OUTPUT_PATH ./build)

set(SOURCE_FILES log.cpp remote.cpp netvar.cpp hack.cpp main.cpp)
add_executable(csgo_external ${SOURCE_FILES})
target_link_libraries(csgo_external "-lX11")

and ran cmake . and make 
This is the log:
william@william-elementarylinux:~$ cd ~/linux-csgo-external-master/
william@william-elementarylinux:~/linux-csgo-external-master$ cmake .
-- The C compiler identification is GNU 4.9.3
-- The CXX compiler identification is GNU 4.9.3
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Configuring done
-- Generating done
-- Build files have been written to: /home/william/linux-csgo-external-master
william@william-elementarylinux:~/linux-csgo-external-master$ make
Scanning dependencies of target csgo_external
[ 20%] Building CXX object CMakeFiles/csgo_external.dir/log.cpp.o
[ 40%] Building CXX object CMakeFiles/csgo_external.dir/remote.cpp.o
[ 60%] Building CXX object CMakeFiles/csgo_external.dir/netvar.cpp.o
[ 80%] Building CXX object CMakeFiles/csgo_external.dir/hack.cpp.o
[100%] Building CXX object CMakeFiles/csgo_external.dir/main.cpp.o
Linking CXX executable build/csgo_external
/usr/bin/ld: cannot find -lX11
collect2: error: ld returned 1 exit status
make[2]: *** [build/csgo_external] Error 1
make[1]: *** [CMakeFiles/csgo_external.dir/all] Error 2
make: *** [all] Error 2
william@william-elementarylinux:~/linux-csgo-external-master$ 

EDIT4: Solved using xiaodongjie's steps and re-installing libx11-dev:i386 as well.

A:

I tried to build it, too.
So I got same error. I found the reason why this error is occurred.
-lX11 is not cflag(compile flag), it's a link flag.
So this flag needs to be in link flags rather than cflags.
https://cmake.org/cmake/help/v3.0/command/target_link_libraries.html
Above page describe about how to set link flags.
So I modified the CMakeLists.txt as follow.
cmake_minimum_required(VERSION 2.8.4)
project(csgo_external)

#removed -lX11 from following line.
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -m32 -std=c++11")
set(EXECUTABLE_OUTPUT_PATH ./build)

set(SOURCE_FILES log.cpp remote.cpp netvar.cpp hack.cpp main.cpp)
add_executable(csgo_external ${SOURCE_FILES})
#appended following line.
target_link_libraries(csgo_external "-lX11")

Also, make sure you have libx11-dev:i386 as well as libx11-dev

