Add -fasm and -std=c99 when supported.

Originally committed as revision 14769 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1125,6 +1125,9 @@ TMPO="${TMPDIR1}/ffmpeg-conf-${RANDOM}-$$-${RANDOM}.o"
 TMPS="${TMPDIR1}/ffmpeg-conf-${RANDOM}-$$-${RANDOM}.S"
 TMPSH="${TMPDIR1}/ffmpeg-conf-${RANDOM}-$$-${RANDOM}.sh"
 
+check_cflags -fasm
+check_cflags -std=c99
+
 case "$arch" in
     i386|i486|i586|i686|i86pc|BePC)
         arch="x86_32"