Simpler way to export HAVE_X_WINDOWS to GDB

* src/.gdbinit: Simplify by removing dependency on globals
implementation.  This is useful for a future performance
improvement that I have in mind.
* src/alloc.c (enum defined_HAVE_X_WINDOWS, defined_HAVE_X_WINDOWS):
New enum and constant.
(gdb_make_enums_visible) [__GNUC__]: Use it, to make
defined_HAVE_X_WINDOWS visible to GDB.
1 job for master in 51 minutes and 32 seconds (queued for 3 seconds)
Status Job ID Name Coverage
failed #1158


Name Stage Failure
test-all Test
Makefile:313: recipe for target 'check-doit' failed
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:288: recipe for target 'check-expensive' failed
make[1]: Leaving directory '/builds/emacs/emacs/test'
make[2]: *** [check-doit] Error 1
make[1]: *** [check-expensive] Error 2
make: *** [check-expensive] Error 2
Makefile:962: recipe for target 'check-expensive' failed
ERROR: Job failed: exit code 1