makekeys: Detach ourselves from X headers entirely

Subsequent to a121b7b0 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>