Add a small comment about a part that should be in configure.

Originally committed as revision 5414 to svn://svn.ffmpeg.org/ffmpeg/trunk

common.mak

@@ -15,6 +15,7 @@ CFLAGS+=-p
 LDFLAGS+=-p
 endif
 
+#FIXME: This should be in configure/config.mak
 ifeq ($(TARGET_ARCH_SPARC64),yes)
 CFLAGS+= -mcpu=ultrasparc -mtune=ultrasparc
 endif