Q:

macOS 'wchar.h' File Not Found

Under OS X 10.9.5 using XCode 6.0.1 I am running into the following issue when I try to run make from the terminal:
fatal error: 'wchar.h' file not found

This issue was not happening prior to a recent upgrade to Mavericks.

A:

First ensure that the latest XCode is installed. Next, we need to install/fix/update Command Line Tools. To do so, run
xcode-select --install

Previously, XCode expected it's active developer directory to be located at
/Applications/Xcode.app/Contents/Developer/

but the latest Command Line Tools is installed to
/Library/Developer/CommandLineTools/

so we need to switch where XCode is looking. To do so, run
sudo xcode-select --switch /Library/Developer/CommandLineTools/

And that should be it. These steps solved the issue for myself and two coworkers.

A:

My error was similar to yours which said string.h not found, I solved it by running this command:

open /Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg

I think that the problem is that for macOS Mojave users, the headers are no longer installed under /usr/include by default.

A:

Adding on ssell's post and peter70's comment, I also still had issues after installing xcode command line tools again, and switching the xcode active developer directory.  I was using CMake, and so deleted the debug/release folders which had existing Makefiles referencing out-of-date locations on Mojave.  So for future problem resolving, after running the commands from ssell's post:
xcode-select --install
sudo xcode-select --switch /Library/Developer/CommandLineTools/
I believe if you've installed the command line tools by installing Xcode itself, you may also need to agree to the licence with sudo xcodebuild --license.  I don't think this is necessary if you used the prior xcode-select command.
Then also delete generated build scripts.  If using CMake, delete the cmake-build-debug and cmake-build-release directories (your names may vary)

