Improper reliance upon pkg-config when C_INCLUDE_PATH is set

[dpryan79]

In setupext.py, matplotlib uses pkg-config --cflags --libs freetype2 to determine where ft2build.h can be found. That works well enough except when that path is already set in either the C_INCLUDE_PATH or CPLUS_INCLUDE_PATH environment variables. If the path is in one of those then pkg-config will not return the appropriate -I/something option and compilation will fail (i.e., setupext.py will mark freetype2 as being absent, even though it's not). pkg-config is behaving correctly, since gcc/clang/etc. check these environment variables.

Would it be possible to have _check_for_pkg_config also check these environment variables. That would solve some issues on Galaxy.

[mdboom]

See #5578 for a solution. I think it makes sense to only look at CPLUS_INCLUDE_PATH, since all of matplotlib's code is C++ (with the exception of the macosx extension), and gcc doesn't seem to look at C_INCLUDE_PATH when in C++ mode (so neither should we).

[dpryan79]

Seems quite reasonable. 👍