Make string constants const. Why is this not default?

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@11835 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

configure

@@ -1785,6 +1785,7 @@ check_cflags -Wpointer-arith
 check_cflags -Wredundant-decls
 check_cflags -Wno-pointer-sign
 check_cflags -Wcast-qual
+check_cflags -Wwrite-strings
 enabled extra_warnings && check_cflags -Winline
 
 # add some linker flags