makekeys: Detach ourselves from X headers entirely

Subsequent to a121b7b0c2 we are no longer
building makekeys with enough -I/foo/bar to find the X11 headers, so if
they're not in a system include path, things fail. Since this utility is
only needed at build time, there's no real reason to demand the X
headers be installed for both the build and target machines if cross-
compiling, we can just assume a vaguely ANSI environment instead.

Tested-by: Niclas Zeising <zeising@daemonic.se>
Reviewed-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>

src/util/makekeys.c

@@ -35,8 +35,10 @@ from The Open Group.
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <stdint.h>
+#include <inttypes.h>
 
-#include "../Xresinternal.h"
+typedef uint32_t Signature;
 
 #define KTNUM 4000