Fix setupext [backport to 1.4.x]

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/.  This also fixes the link in `INSTALL` which was malformed. 

This addresses #3509.
I don't think this fits in the spirit of what this function does. It is checking for pkg-config data and not if the header exists. The function which calls this has the responsibility of doing any additional checks (like looking for a header file if that is the correct behavior for the particular dependency).