Merge pull request matplotlib#3503 from jbmohler/win_fix_simple

BLD : restore non-searching for libpng on windows

Author: tacaswell

INSTALL

@@ -319,4 +319,4 @@ Framework with Visual Studio).  In addition to Visual Studio `CMake
 
 Since there is no canonical Windows package manager the build methods for
 freetype, zlib, libpng, tcl, & tk source code are documented as a build script
-at `matplotlib-winbuild <https://github.com/jbmohler/matplotlib-winbuild>`.
+at `matplotlib-winbuild <https://github.com/jbmohler/matplotlib-winbuild>`_.

setupext.py

@@ -1003,6 +1003,9 @@ class Png(SetupPackage):
     name = "png"
 
     def check(self):
+        if sys.platform == 'win32':
+            return "Unknown version"
+
         status, output = getstatusoutput("libpng-config --version")
         if status == 0:
             version = output