Warn if a comparison is always true or always false due to the

limited range of the data type.
The only new warning this causes (if i didnt miss one) is:
udp.c:272: warning: comparison of unsigned expression < 0 is always false
which is a real bug

Originally committed as revision 11839 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1786,6 +1786,7 @@ check_cflags -Wredundant-decls
 check_cflags -Wno-pointer-sign
 check_cflags -Wcast-qual
 check_cflags -Wwrite-strings
+check_cflags -Wtype-limits
 enabled extra_warnings && check_cflags -Winline
 
 # add some linker flags