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.

c++tools/Makefile.in

@@ -22,6 +22,7 @@ libexecdir := @libexecdir@
 target_noncanonical := @target_noncanonical@
 gcc_version := $(shell @get_gcc_base_ver@ $(srcdir)/../gcc/BASE-VER)
 libexecsubdir := $(libexecdir)/gcc/$(target_noncanonical)/$(gcc_version)
+INSTALL := @INSTALL@
 INSTALL_PROGRAM := @INSTALL_PROGRAM@
 INSTALL_STRIP_PROGRAM := $(srcdir)/../install-sh -c -s
 AUTOCONF := @AUTOCONF@
@@ -41,13 +42,14 @@ all::
 mostlyclean::
 	rm -f $(MAPPER.O)
 
-clean::
+clean:: mostlyclean
 	rm -f g++-mapper-server$(exeext)
 
-distclean::
-	rm -f config.log config.status config.h
+distclean:: clean
+	rm -f config.log config.status config.h config.cache Makefile
+	rm -f $(MAPPER.O:%.o=%.d)
 
-maintainer-clean::
+maintainer-clean:: distclean
 
 install::
 
@@ -132,6 +134,6 @@ config.h: config.status config.h.in
 config.status: $(srcdir)/configure $(srcdir)/config.h.in
 	if test -x $@; then ./$@ -recheck; else $< @configure_args@; fi
 
-.PHONY: all check clean distclean maintainer-clean
+.PHONY: all check mostlyclean clean distclean maintainer-clean
 
 -include $(MAPPER.O:.o=.d)