Use m4_include, not sinclude in .m4 files

Pedro pointed out that sinclude does not error if a file is missing.
This patch changes gdb to only use m4_include, which seems more
correct.

gdb/ChangeLog
2019-10-23  Tom Tromey  <tom@tromey.com>

	* configure: Rebuild.
	* acinclude.m4: Use m4_include, not sinclude.

gdb/gdbserver/ChangeLog
2019-10-23  Tom Tromey  <tom@tromey.com>

	* configure: Rebuild.
	* acinclude.m4: Use m4_include, not sinclude.

gdb/testsuite/ChangeLog
2019-10-23  Tom Tromey  <tom@tromey.com>

	* configure: Rebuild.
	* aclocal.m4: Use m4_include, not sinclude.

Change-Id: I970362e0af7875f9f72796401126acf0ff6dba11