Provide win32-based dlapi replacements on windows platforms without dlfcn.h.

Provide win32-based dlapi replacements on windows platforms without dlfcn.h.

ld/ChangeLog:

	* configure.in: If <dlfcn.h> can't be found, try for <Windows.h>
	* configure: Regenerate.
	* config.in: Likewise.
	* plugin.c [!HAVE_DLFCN_H && HAVE_WINDOWS_H] (dlopen): Provide
	trival LoadLibrary-based replacement for Windows systems.
	[!HAVE_DLFCN_H && HAVE_WINDOWS_H] (dlsym): Likewise trivial
	replacement based on GetProcAddress.
	[!HAVE_DLFCN_H && HAVE_WINDOWS_H] (dlsym): Likewise FreeLibrary.
	* sysdep.h: Don't infer presence of <dlfcn.h> from ENABLE_PLUGINS
	anymore, use its own guard.