winsup/doc: Make it easier to extend xidepend to more targets

winsup/doc: Make it easier to extend xidepend to more targets

Change xidepend to create a variable containing all the XIncluded sources, which
can be used as a dependency, rather than writing the dependency target itself.

Future work: Makefile.dep should depend on xidepend, but xidepend should not be
passed to itself.

2015-06-12  Jon Turney  <jon.turney@dronecode.org.uk>

	* xidepend: Write a Makefile fragment defining variables
	containing all the XIncluded sources, rather than a dependency on
	those sources.
	* Makefile.in: Use that variable to express the dependency.

Signed-off-by: Jon TURNEY <jon.turney@dronecode.org.uk>