* configure.ac: Use CPPFLAGS instead of CFLAGS for libpng.
Patch by Sebastian Reitenbach <sebastia@l00-bugdead-prods.de>


git-svn-id: svn+ssh://svn.gna.org/svn/gnustep/libs/gui/trunk@36268 72102866-910b-0410-8b05-ffd578937521