[tlbuild] Late error on missing X11 headers

[Mojca Miklavec]

Hi,

When trying to build TeX Live on RPi I hit the following error:

mv -f $depbase.Tpo $depbase.Po
../../../utils/xpdfopen/xpdfopen-src/sendx.c:29:22: fatal error:
X11/Xlib.h: No such file or directory
 #include <X11/Xlib.h>
                      ^
compilation terminated.

It would be great if this at least failed during the configure step.

Mojca