Add -Wmissing-prototypes to CFLAGS if available.

Originally committed as revision 22009 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -2699,6 +2699,7 @@ check_cflags -Wcast-qual
 check_cflags -Wwrite-strings
 check_cflags -Wtype-limits
 check_cflags -Wundef
+check_cflags -Wmissing-prototypes
 enabled extra_warnings && check_cflags -Winline
 
 # add some linker flags