Add DIRECTDRAW_CFLAGS and DIRECTSOUND_CFLAGS to Makefile.am; save and restore the various flags in the directdraw/dir...

Original commit message from CVS:
* configure.ac:
* sys/directdraw/Makefile.am:
* sys/directsound/Makefile.am:
Add DIRECTDRAW_CFLAGS and DIRECTSOUND_CFLAGS to Makefile.am; save
and restore the various flags in the directdraw/directsound
detection section. Apparently improves cross-compiling for win32
with mingw32 under some circumstances (#437539).