detection/configuration, DDX options. Make building of Xorg DDX fully
optional. Clarify and correct some help texts. Change all comments to
use dnl instead of #. Quote all tests correctly, and guard
pure-variable tests with 'x' (e.g. test $DMX = yes -> test "x$DMX" =
xyes). Since the DDXes seem to have pretty divergent extension support
these days, get rid of EXTENSION_LIBS, DMX_EXTENSIONS and
XPRINT_EXTENSIONS, and go back to building extension lists by hand in
the DDX-specific sections. Use portable POSIX constructs everywhere
(e.g. test foo && test bar, instead of test foo -a bar).
Clean up old cruft.
Set _XSERVER64 on 64-bit architectures, and use x86_64 for host_cpu instead
of amd64 (Jürg Billeter).
