Do without ld ENABLE_PLUGINS

Do without ld ENABLE_PLUGINS

Instead, use BFD_SUPPORTS_PLUGINS.

	* ldfile.c: Replace uses of ENABLE_PLUGINS with BFD_SUPPORTS_PLUGINS.
	* ldlang.c: Likewise.
	* ldlang.h: Likewise.
	* ldlex.h: Likewise.
	* ldmain.c: Likewise.
	* lexsup.c: Likewise.
	* plugin.c: Wrap body of file in #if BFD_SUPPORTS_PLUGINS.
	* testplug.c: Likewise.
	* testplug2.c: Likewise.
	* testplug3.c: Likewise.
	* testplug4.c: Likewise.
	* configure.ac (ENABLE_PLUGINS): Don't define AM_CONTITIONAL.
	* Makefile.am: Remove ENABLE_PLUGINS conditionals.
	(PLUGIN_CFLAGS): Don't define.
	(PLUGIN_C, PLUGIN_H, PLUGIN_OBJECT): Likewise.  Substitute all
	uses with plugin file name.
	* configure: Regenerate.
	* Makefile.in: Regenerate.