Merge the ./config directory between GCC and Binutils

This pulls in the missing Binutils pieces into the ./config
directory. Contains these missing Binutils changes:

2014-08-14  Alan Modra  <amodra@gmail.com>

	* plugins.m4: Test for dlfcn.h or windows.h here to set default
	for --enable-plugins.  Report error if someone tries to enable
	plugins on a host we don't support.

2014-08-19  Alan Modra  <amodra@gmail.com>

	* plugins.m4 (AC_PLUGINS): If plugins are enabled, add -ldl to
	LIBS via AC_SEARCH_LIBS.

For this commit:

2014-11-26  Jan-Benedict Glaw  <jbglaw@lug-owl.de>

	* config/plugins.m4: Merge from Binutils.

From-SVN: r218102

config/plugins.m4

 AC_DEFUN([AC_PLUGINS],
 [
-AC_ARG_ENABLE([plugins],
-AS_HELP_STRING([--enable-plugins], [Enable support for plugins (defaults no)]),
-[case "${enableval}" in
-  yes | "") plugins=yes ;;
-  no) plugins=no ;;
-  *) plugins=yes ;;
- esac],
-[plugins=no])
+  maybe_plugins=no
+  AC_CHECK_HEADERS([dlfcn.h], [maybe_plugins=yes], [], [AC_INCLUDES_DEFAULT])
+  AC_CHECK_HEADERS([windows.h], [maybe_plugins=yes], [], [AC_INCLUDES_DEFAULT])
+
+  AC_ARG_ENABLE([plugins],
+    AS_HELP_STRING([--enable-plugins], [Enable support for plugins]),
+    [case "${enableval}" in
+      no) plugins=no ;;
+      *) plugins=yes
+         if test "$maybe_plugins" != "yes" ; then
+	   AC_MSG_ERROR([Building with plugin support requires a host that supports dlopen.])
+	 fi ;;
+     esac],
+    [plugins=$maybe_plugins]
+  )
+  if test "$plugins" = "yes"; then
+    AC_SEARCH_LIBS([dlopen], [dl])
+  fi
 ])