Add an actual 3dnow intrinsinc so the configure script doesn't just check for the existence of the 3dnow header

Originally committed as revision 5383 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1110,6 +1110,7 @@ fi
 cat > $TMPC << EOF
 #include <mm3dnow.h>
 int main(void) {
+_m_femms();
 return 0;
 }
 EOF