Fix automatic dependency tracking

Fix automatic dependency tracking

Commit f38307f5 changed COMPILE.post and POSTCOMPILE to remove
$(basename) from the dependency file name computation.  However, it
did not update the `-include' at the end of the Makefile.in; this in
effect disabled automatic dependency tracking.

This patch restores the $(basename) wrapper so that the dependency
files are named "file.Po" rather than "file.o.Po".

I also tested the non-gcc3 dependency mode, which pointed out that
this case hadn't been working since the switch to C++.  This is also
fixed in this patch.

Tested by rebuilding.

ChangeLog
2017-10-09  Tom Tromey  <tom@tromey.com>

	* Makefile.in (COMPILE.post, POSTCOMPILE): Restore $(basename).
	(COMPILE.pre): Use $(CXX).