build: Use -include instead of conditional include.

Automake and GNU Make both use the endif keyword, which conflicts and
elicits an error for matching if/ifdef and endif.

This patch changes the conditional include to use "-include" to prevent
a warning about a possible empty tmake_file.

libgomp/ChangeLog

2020-06-22  David Edelsohn  <dje.gcc@gmail.com>

	* Makefile.am: Use -include.
	* Makefile.in: Regenerate.

libatomic/ChangeLog

2020-06-22  David Edelsohn  <dje.gcc@gmail.com>

	* Makefile.am: Use -include.
	* Makefile.in: Regenerate.

libstdc++-v3/ChangeLog

2020-06-22  David Edelsohn  <dje.gcc@gmail.com>

	* Makefile.am: Use -include.
	* Makefile.in: Regenerate.

libgfortran/ChangeLog

2020-06-22  David Edelsohn  <dje.gcc@gmail.com>

	* Makefile.am: Use -include.
	* Makefile.in: Regenerate.