Use top-level config support for enabling plugins.

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.

diff --git a/gold/aclocal.m4 b/gold/aclocal.m4
index 47cb21fc5a..7742a0e245 100644
--- a/gold/aclocal.m4
+++ b/gold/aclocal.m4
@@ -1007,6 +1007,7 @@ m4_include([../config/lead-dot.m4])
 m4_include([../config/nls.m4])
 m4_include([../config/override.m4])
 m4_include([../config/po.m4])
+m4_include([../config/plugins.m4])
 m4_include([../config/progtest.m4])
 m4_include([../config/zlib.m4])
 m4_include([../bfd/warning.m4])
diff --git a/gold/configure.ac b/gold/configure.ac
index 5ba43f2f95..1ed6c0d45c 100644
--- a/gold/configure.ac
+++ b/gold/configure.ac
@@ -28,6 +28,8 @@ AM_INIT_AUTOMAKE([no-dist parallel-tests])
 
 AM_CONFIG_HEADER(config.h:config.in)
 
+AC_USE_SYSTEM_EXTENSIONS
+
 # PR 14072
 AH_VERBATIM([00_CONFIG_H_CHECK],
 [/* Check that config.h is #included before system headers
@@ -100,14 +102,7 @@ if test "$threads" = "yes"; then
 fi
 AM_CONDITIONAL(THREADS, test "$threads" = "yes")
 
-AC_ARG_ENABLE([plugins],
-[  --enable-plugins        linker plugins],
-[case "${enableval}" in
-  yes | "") plugins=yes ;;
-  no) plugins=no ;;
-  *) plugins=yes ;;
- esac],
-[plugins=no])
+AC_PLUGINS
 if test "$plugins" = "yes"; then
   AC_DEFINE(ENABLE_PLUGINS, 1,
 	    [Define to enable linker plugins])
diff --git a/gold/options.cc b/gold/options.cc
index f511ba1e79..05b9bba5fa 100644
--- a/gold/options.cc
+++ b/gold/options.cc
@@ -448,7 +448,6 @@ General_options::parse_library(const char*, const char* arg,
   cmdline->inputs().add_file(file);
 }
 
-#ifdef ENABLE_PLUGINS
 void
 General_options::parse_plugin(const char*, const char* arg,
 			      Command_line*)
@@ -464,7 +463,6 @@ General_options::parse_plugin_opt(const char*, const char* arg,
 {
   this->add_plugin_option(arg);
 }
-#endif // ENABLE_PLUGINS
 
 void
 General_options::parse_R(const char* option, const char* arg,
@@ -1208,6 +1206,13 @@ General_options::finalize()
 		 program_name);
 #endif
 
+#ifndef ENABLE_PLUGINS
+  if (this->has_plugins())
+    gold_fatal(_("cannot use --plugin: "
+		 "%s was compiled without plugin support"),
+	       program_name);
+#endif
+
   std::string libpath;
   if (this->user_set_Y())
     {
diff --git a/gold/options.h b/gold/options.h
index e75a8bd452..98d6be8009 100644
--- a/gold/options.h
+++ b/gold/options.h
@@ -1125,6 +1125,12 @@ class General_options
 		 N_("Load a plugin library"), N_("PLUGIN"));
   DEFINE_special(plugin_opt, options::TWO_DASHES, '\0',
 		 N_("Pass an option to the plugin"), N_("OPTION"));
+#else
+  DEFINE_special(plugin, options::TWO_DASHES, '\0',
+		 N_("Load a plugin library (not supported)"), N_("PLUGIN"));
+  DEFINE_special(plugin_opt, options::TWO_DASHES, '\0',
+		 N_("Pass an option to the plugin (not supported)"),
+		 N_("OPTION"));
 #endif
 
   DEFINE_bool(posix_fallocate, options::TWO_DASHES, '\0', true,