Only set HAVE_MMX when a supported CPU is specified

Originally committed as revision 21277 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1789,8 +1789,12 @@ case $cpu in
         cpuflags="-mcpu=cell"
         enable ldbrx
     ;;
+    i[345]86|pentium)
+        cpuflags="-march=$cpu"
+        disable mmx
+    ;;
     # targets that do NOT support conditional mov (cmov)
-    i[345]86|pentium|pentium-mmx|k6|k6-[23]|winchip-c6|winchip2|c3)
+    pentium-mmx|k6|k6-[23]|winchip-c6|winchip2|c3)
         cpuflags="-march=$cpu"
         disable cmov
     ;;