This commit allows users to specify a path to their "etags"
executable for use when doing "make tags".
I based this patch off of this one from upstream automake:
https://git.savannah.gnu.org/cgit/automake.git/commit/m4?id=d2ccbd7eb38d6a4277d6f42b994eb5a29b1edf29
This means that I just supplied variables that the user can override
for the tags programs, rather than having the configure scripts
actually check for them. I handle etags and ctags separately because
the intl subdirectory has separate targets for them. This commit
only affects the subdirectories that use handwritten Makefiles; the
ones that use automake will have to wait until we update the version
of automake used to be 1.16.4 or newer before they'll be fixed.

Addresses #103021

gcc/ChangeLog:

	PR other/103021
	* Makefile.in: Substitute CTAGS, ETAGS, and CSCOPE
	variables. Use ETAGS variable in TAGS target.
	* configure: Regenerate.
	* configure.ac: Allow CTAGS, ETAGS, and CSCOPE
	variables to be overridden.

gcc/ada/ChangeLog:

	PR other/103021
	* gcc-interface/Make-lang.in: Use ETAGS variable in
	TAGS target.

gcc/c/ChangeLog:

	PR other/103021
	* Make-lang.in: Use ETAGS variable in TAGS target.

gcc/cp/ChangeLog:

	PR other/103021
	* Make-lang.in: Use ETAGS variable in TAGS target.

gcc/d/ChangeLog:

	PR other/103021
	* Make-lang.in: Use ETAGS variable in TAGS target.

gcc/fortran/ChangeLog:

	PR other/103021
	* Make-lang.in: Use ETAGS variable in TAGS target.

gcc/go/ChangeLog:

	PR other/103021
	* Make-lang.in: Use ETAGS variable in TAGS target.

gcc/objc/ChangeLog:

	PR other/103021
	* Make-lang.in: Use ETAGS variable in TAGS target.

gcc/objcp/ChangeLog:

	PR other/103021
	* Make-lang.in: Use ETAGS variable in TAGS target.

intl/ChangeLog:

	PR other/103021
	* Makefile.in: Use ETAGS variable in TAGS target,
	CTAGS variable in CTAGS target, and MKID variable
	in ID target.
	* configure: Regenerate.
	* configure.ac: Allow CTAGS, ETAGS, and MKID
	variables to be overridden.

libcpp/ChangeLog:

	PR other/103021
	* Makefile.in: Use ETAGS variable in TAGS target.
	* configure: Regenerate.
	* configure.ac: Allow ETAGS variable to be overridden.

libiberty/ChangeLog:

	PR other/103021
	* Makefile.in: Use ETAGS variable in TAGS target.
	* configure: Regenerate.
	* configure.ac: Allow ETAGS variable to be overridden.