Simpler way to export HAVE_X_WINDOWS to GDB

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.