Author is Peter Breitenlohner <peb@mppmu.mpg.de>
Bug #17946, attachment #19439

Define as 1 (one) as done by autoconf and the command line
option, e.g. -DX11_t, not as empty.

This avoids the gcc (3.4.6) warnings:
	../../libX11-1.1.5/src/x11_trans.c:27:1: warning: "X11_t" redefined
	<command line>:7:1: warning: this is the location of the previous definition
	../../libX11-1.1.5/src/x11_trans.c:28:1: warning: "TRANS_CLIENT" redefined
	<command line>:8:1: warning: this is the location of the previous definition

Similarly, follow the autoconf convention to define XTHREADS
and XUSE_MTSAFE_API as one.

This avoids analogous warnings when compiling libXcomposite,
libXcursor, and libXdamage.

No reason to AC_SUBST XTHREADS and XUSE_MTSAFE_API (unused).