Also recognizes --plugin options when plugins are disabled.

2018-03-28  Cary Coutant  <ccoutant@gmail.com>

gold/
	PR gold/21423
	PR gold/22500
	* configure.ac: Call AC_USE_SYSTEM_EXTENSIONS.
	Replace check for --enable-plugins with AC_PLUGINS.
	* options.cc (parse_plugin, parse_plugin_opt): Remove #ifdef.
	(General_options::finalize): Check if plugins enabled.
	* options.h (--plugin, --plugin-opt): Define even if plugins not
	enabled.
	* Makefile.in: Regenerate.
	* aclocal.m4: Regenerate.
	* configure: Regenerate.
	* testsuite/Makefile.in: Regenerate.