To enable SSE support in libpng, manually edit configure.ac and Makefile.am,
following the instructions in the configure.ac.patch and Makefile.am.patch
files, then configure with -DPNG_INTEL_SSE in CPPFLAGS.

If you have moved the *.c files to a different directory, be sure to update
the '#include "../../pngpriv.h"' line in both files if necessary to point
to the correct relative location of pngpriv.h.