Define mm_flags/support to be 0 on architectures where they don't exist.

Make sure we don't try to set them if that's the case.
This lets dct-test and fft-test build with --disable-mmx on x86, but not yet on ARM.

Originally committed as revision 13957 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/dct-test.c

@@ -531,7 +531,10 @@ int main(int argc, char **argv)
 
     init_fdct();
     idct_mmx_init();
+
+#ifndef mm_flags
     mm_flags = mm_support();
+#endif
 
     for(i=0;i<256;i++) cropTbl[i + MAX_NEG_CROP] = i;
     for(i=0;i<MAX_NEG_CROP;i++) {