Fix setupext

Allows for installation on Windows.  The lack of `getstatusoutput` caused build errors.  `getstatusoutput` in including on Windows on Python 2 and 3.3.4+.  Also, `Png` was unable to locate `png.h` because `has_include_file` did not respect the `INCLUDE` environmental variable.

With these changes I was able to build matplotlib on Windows 7, Python 2.7.8 on 32bit Anaconda, using https://github.com/jbmohler/matplotlib-winbuild/.