Add proper search for libpng and freetype on win32 during install.

Allow for getstatusoutput on win32 and use INCLUDE and LIB env vars

Back off on the use of the environmental variables

Pinpointed necessary location for INCLUDE

Fix link to matplotlib-winbuild

Normalize handling of library location finding

Include more paths in the search

Restore _check for_pkg_config behavior

Implement getstatusoutput on win32

Remove win32 getstatusoutput in favor of explicit check_include_file
Merge pull request #3510 from blink1073/fix-setupext

BLD : improve setupext.py on windows