c++tools: Fix Makefile to properly clean and rebuild [PR101834]

The c++tools makefile doesn't remove progressively more files in each of
mostlyclean, clean, and distclean. Instead, each removes a different set
of files (and some files are not removed by any target). Use
prerequisites so that everything is removed.

Also, building in the $objdir/c++tools directory doesn't work, because
the INSTALL variable is never set. It works when building from the
top-level because INSTALL is set in the environment when recursively
invoking make for sub-directories.

c++tools/ChangeLog:

	PR bootstrap/101834
	* Makefile.in (INSTALL): Set variable.
	(mostlyclean): Mark as a phony target.
	(clean): Add mostlyclean as a prerequisite.
	(distclean): Add clean as a prerequisite and remove more files.
	(maintainer-clean): Add distclean as a prerequisite.