Add 3dnow intrinsic support for AMD64 machines: -march=athlon means IA32 for...

Add 3dnow intrinsic support for AMD64 machines: -march=athlon means IA32 for GCC, so use -m3dnow instead.
Note that there's no way to specificly activate 3dnow ext support

Originally committed as revision 5380 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1115,7 +1115,7 @@ return 0;
 EOF
 
 mm3dnow=no
-if $cc -march=athlon -o $TMPO $TMPC 2> /dev/null ; then
+if $cc -m3dnow -o $TMPO $TMPC 2> /dev/null ; then
   mm3dnow=yes
 fi
 

libavcodec/Makefile

@@ -350,7 +350,7 @@ depend: CFLAGS+= -msse
 endif
 ifdef TARGET_BUILTIN_3DNOW
 i386/fft_3dn.o: CFLAGS+= -m3dnow
-i386/fft_3dn2.o: CFLAGS+= -march=athlon
+i386/fft_3dn2.o: CFLAGS+= -m3dnow
 endif
 endif