c++tools: fix install-strip [PR 98328]

I'd missed an install-strip rule in c++tools.  Here it is, cribbed
from gcc/ subdir.

	c++tools/
	* Makefile.in (INSTALL): Replace with ...
	(INSTALL_PROGRAM): ... this.
	(INSTALL_STRIP_PROGRAM): New.
	(install-strip): New target.
	(install): Use INSTALL_PROGRAM.
	* configure.ac: Add INSTALL_PROGRAM.
	* configure: Regenerated.