Mixing declarations and statements is an error, so is any construct

depending on sizeof(void) (like void_ptr++)

Originally committed as revision 11798 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1785,6 +1785,8 @@ check_cflags -Wpointer-arith
 check_cflags -Wredundant-decls
 check_cflags -Wno-pointer-sign
 check_cflags -Wcast-qual
+check_cflags -Werror=declaration-after-statement
+check_cflags -Werror=pointer-arith
 enabled extra_warnings && check_cflags -Winline
 
 # add some linker flags