include: Add RELOCATE_PROJECTROOT to xwin-config.h header

RELOCATE_PROJECTROOT is AC_DEFINED in configure.ac, but currently has no effect
as it doesn't appear in any AC_CONFIG_HEADER header.

When packaged for Windows, we do not have a unix-style filesystem tree, where
file needed by the X server can be found in fixed, absolute paths under the
prefix (PROJECTROOT).

Instead, the filesystem tree containing files needed by the X server and clients
will be installed with the directory containing the X server executable as the
root directory of that tree.

(Typically, this will be in the Program Files directory, which does not have a
fixed name, as it can be moved, localized, or added to to indicate x86 or x64
binaries)

So, RELOCATE_PROJECTROOT is used to make a native Windows build of the X server
look for various files (fonts, xkb data) in locations relative to the X server
rather than at absolute paths, by translating those paths at run-time.

Additionally the XKEYSYMDB, XERRORDB, XLOCALEDIR env vars checked by libX11 are
set appropriately for clients started by the X server.

Signed-off-by: Ryan Pavlik <rpavlik@iastate.edu>
Reviewed-by: Jon TURNEY <jon.turney@dronecode.org.uk>
Reviewed-by: Colin Harrison <colin.harrison@virgin.net>

include/xwin-config.h.in

@@ -31,3 +31,6 @@
 
 /* Default log location */
 #undef DEFAULT_LOGDIR
+
+/* Whether we should re-locate the root to where the executable lives */
+#undef RELOCATE_PROJECTROOT