Warn about casts which remove const.

Feel free to flame me if you disagree that such code is wrong.

Originally committed as revision 11699 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1784,6 +1784,7 @@ check_cflags -Wdisabled-optimization
 check_cflags -Wpointer-arith
 check_cflags -Wredundant-decls
 check_cflags -Wno-pointer-sign
+check_cflags -Wcast-qual
 enabled extra_warnings && check_cflags -Winline
 
 # add some linker flags