I'm pretty sure that I wrote the code Ned refers to, and that's indeed only targeting darwin to avoid breaking other platforms. That code could easily be made actively globally though, the only reason I didn't do so at the time is we were still getting used to the fact that MacOS used the same build infrastructure as other major platforms and hence were overly cautious.

Note that this doesn't do the same thing as Jan requests: GCC[1] and clang[2] can add additional directories to their header file search path using environment variables. 

For Python's setup.py file the following are important:

* C_INCLUDE_PATH: a list of directories that are handled as if they are present at the end of the list of -isystem options (that is, system include files at a lower priority that the user provided ones)

* CPATH: simular, but for the '-I' option

* LIBRARY_PATH: similar, but for the '-L' option

All of them have a syntax similar to $PATH.

A patch to add support for these variables (and enables the handling of -I and -L for other platforms than darwin) should be easy enough, but I agree with Ned that this would be a new feature because it could break existing build systems (that is, building Python with this patch could result in a different build than without the patch due to the build picking up more or different external libraries).

Jan: are you willing to write such a patch? And if so, are you willing to sign a contributor agreement?

[1]: http://gcc.gnu.org/onlinedocs/gcc/Environment-Variables.html
[2]: http://clang.llvm.org/doxygen/Tools_8cpp_source.html