[tlbuild] Disabling X11 (on macOS)

[Mojca Miklavec]

Hi,

I need some more time to investigate, but I find it strange that
building TeX Live on Mac OS X 10.6 against a specific SDK correctly
builds TeX Live, but for some reason excludes X11 (X11 is bundled with
OS, but apparently missing in the SDK?), while doing so on one of the
latest macOS versions with no X11 package installed separately breaks
while building xpdfopen:

checking for X... no
...
../../../utils/xpdfopen/xpdfopen-src/sendx.c:29:10: fatal error:
'X11/Xlib.h' file not found
#include <X11/Xlib.h>
         ^~~~~~~~~~~~
1 error generated.
make[3]: *** [xpdfopen-src/sendx.o] Error 1


Sample build logs:
10.6 https://build.contextgarden.net/#/builders/59/builds/9 (works)
10.13 https://build.contextgarden.net/#/builders/53/builds/8 (fails)

I'm pretty sure that either adding something like "--without-x" flag
or installing X11 would get the build to succeed, but it would be nice
if the complex autofoo machinery could detect the absence of X11 and
automatically disable building those packages that require it. (It
apparently does so to some extent in the case of 10.6.)

Mojca