gdb: Support install-strip target

gdb/
	* configure.ac: Call AM_PROG_INSTALL_STRIP.
	* configure: Regenerate.
	* aclocal.m4: Regenerate.
	* Makefile.in (install_sh, INSTALL_STRIP_PROGRAM, STRIP):
	New substituted	variables.
	(install-strip): New target.
	(INSTALL_SCRIPT): New substituted variable.
	(FLAGS_TO_PASS): Add it.
	(install-only): Use $(INSTALL_SCRIPT) rather than
	$(INSTALL_PROGRAM) for gcore.

(cherry picked from commit 4869db5e97b2062a3adf81158577171990248ef3)
Automatic date update in version.in