zlib recipe overwrites ENV settings
I'm trying to build zlib using the following profile
```
[settings]
os=Linux
os_build=Linux
arch=x86_64
arch_build=x86_64
compiler=clang
compiler.version=6.0
compiler.libcxx=libc++

[env]
CC = clang-6.0
CXX = clang++-6.0
```

Building zlib fails because I don't have _clang_ binary but only _clang-6.0_ binary because it's overwritten by the recipe. You also can't build with a different clang version, only the one provided by your _clang_ binary.

https://github.com/conan-io/conan-center-index/blob/a025bb0e4d523db2cf08c4eb7730079c94368b8e/recipes/zlib/1.2.11/conanfile.py#L68

It was already metioned in #306 but it's still in the recipe.
diff --git a/recipes/zlib/1.2.11/conanfile.py b/recipes/zlib/1.2.11/conanfile.py
index 89f67af9c..370eefaff 100644
--- a/recipes/zlib/1.2.11/conanfile.py
+++ b/recipes/zlib/1.2.11/conanfile.py
@@ -65,7 +65,9 @@ class ZlibConan(ConanFile):
         else:
             make_target = "libz.a"
 
-        env = {"CC": "clang", "CXX": "clang++"} if "clang" in str(self.settings.compiler) else {}
+        env = {}
+        if "clang" in str(self.settings.compiler) and tools.get_env("CC") is None and tools.get_env("CXX") is None:
+            env = {"CC": "clang", "CXX": "clang++"}
         with tools.environment_append(env):
             env_build.configure("../", build=False, host=False, target=False)
             env_build.make(target=make_target)
@@ -80,7 +82,7 @@ class ZlibConan(ConanFile):
 
     def _build_zlib(self):
         with tools.chdir(self._source_subfolder):
-            # https://github.com/madler/zlib/issues/268 
+            # https://github.com/madler/zlib/issues/268
             tools.replace_in_file('gzguts.h',
                                   '#if defined(_WIN32) || defined(__CYGWIN__)',
                                   '#if defined(_WIN32) || defined(__MINGW32__)')
['recipes/zlib/1.2.11/conanfile.py']