Make string constants const. Why is this not default?

Originally committed as revision 11835 to svn://svn.ffmpeg.org/ffmpeg/trunk

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