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>

winsup/doc/xidepend

@@ -16,7 +16,7 @@ do
 	then
 		# This file uses XIncludes.  Let's chase its deps recursively.
 		base=`basename "$f" .xml`
-		if [ $subproc -eq 0 ] ; then echo -n "$base/$base.html $base/$base.pdf:" ; fi
+		if [ $subproc -eq 0 ] ; then echo -n "${base}_SOURCES=${f}" ; fi
 
 		deps=`grep 'xi:include.*href' "$f" | cut -f2 -d\" | tr '\n' ' '`
 		echo -n " $deps"