Import AC_CHECK_DECL for C++ fixes from git Autoconf.

config/:
	PR bootstrap/42798
	* override.m4 (_AC_CHECK_DECL_BODY, _AC_CHECK_DECLS): Import
	definitions from git Autoconf.

Co-Authored-By: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>

From-SVN: r160210