Don't use -msse when building pixman-mmx.c

This prevents gcc 4.2 from generating SSE instructions for its builtin
functions, the X server from going SIGILL on CPUs supporting MMX but not SSE,
and us from losing (closes: #442829).

Also enable maintainer mode in configure.ac so we can carry the patch to
configure directly in the diff.gz without running autoreconf at build time.