After I opened PR10296 for LIBRARY_PATH not being honored and PR10297 for CPATH not
being honored, I noticed with the proposed
solutions in
   Is there a reason why the fourth option...

4. Leave the environment variable inclusion in the Frontend, and just replace
the FIXME at CompilerInvocation.cpp:1330 with the following:

  Opts.EnvIncPath = ::getenv("CPATH");
  Opts.CEnvIncPath = ::getenv("C_INCLUDE_PATH");
  Opts.ObjCEnvIncPath = ::getenv("OBJC_INCLUDE_PATH");
  Opts.CXXEnvIncPath = ::getenv("CPLUS_INCLUDE_PATH");
  Opts.ObjCXXEnvIncPath = ::getenv("OBJCPLUS_INCLUDE_PATH");

...hasn't been implemented? This would need to be expanded to solve PR10296 for LIBRARY_PATH
and the null checks added of course.