Moved the check for the SWIG environment variable from darwin-config.py to Sconstruct so all platforms can use it. You should now be able to specify CXXFLAGS either by command line or by setting your CXXFLAGS environment variable.

prefix=@prefix@
exec_prefix=@exec_prefix@
libdir=@libdir@
includedir=@includedir@/libpng12

Name: libpng
Description: Loads and saves PNG files
Version: 1.2.29
Libs: -L${libdir} -lpng12
Cflags: -I${includedir}