Supporting search paths for headers and libraries

gcc and clang honor C_INLCUDE_PATH CPLUS_INCLUDE_PATH (OBJC_INCLUDE_PATH and OBJCPLUS_INCLUDE_PATH) environment variables to search for c,cxx, (objc, objcxx) headers.

Similarly LIBRARY_PATH can be used to specify a search path for libraries.

EDIT: For msvc appearently there are INLCUDE and LIB environment variables doing the same thing.

What problem will this solve?

A header that is in C_INCLUDE_PATH or a library in LIBRARY_PATH can be found by the compiler/linker.

os.findheader and os.findlib will currently fail to find them.

What might be a solution?

Extend os.findheader and os.findlib to recognise these values.

What other alternatives have you already considered?

Rather than searching through a list of search paths manually let the compiler/linker do it with small test programs.

Anything else we should know?
Add any other context or screenshots about the feature request here.

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Start at the os.findheader and os.findlib entry points. Review how they currently search paths, then compare handling of C_INCLUDE_PATH, CPLUS_INCLUDE_PATH, OBJC_INCLUDE_PATH, OBJCPLUS_INCLUDE_PATH, LIBRARY_PATH, and MSVC's INCLUDE/LIB variables. Done means headers and libraries in those paths are found for the relevant compiler environments.