[tlbuild] Disabling X11 (on macOS)

[Karl Berry]

    ../../../utils/xpdfopen/xpdfopen-src/sendx.c:29:10: fatal error:
    'X11/Xlib.h' file not found

I understand the desire for automatic adaptation, but we have never
tried to tinker with automatically disabling xpdfopen and mf-with-x (=
what gets configured) based on the current system before, and I don't
much want to start now. It's not clear to me that it is desirable,
because there's no way to know if the resulting silent failure to
compile is expected. I think specifying --without-x when needed is the
way to go.

    10.6 https://build.contextgarden.net/#/builders/59/builds/9 (works)

Looking at the stdio log, that system evidently has enough X (namely
Xlib) to build xpdfopen.
checking for X... libraries /usr/X11/lib, headers 
..
and the compilation of sendx.c succeeds.

    automatically disable building those packages that require it.
    (It apparently does so to some extent in the case of 10.6.)

There are no such system/version-dependent special cases that I know of.

The X-dependent programs specify the [x] option to KPSE_ENABLE_PROG, and
that disables them when --without-x is given. Otherwise, AC_PATH_XTRA is
called and the results for includes and libraries are used. -k