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.

ld/plugin.c

@@ -32,6 +32,9 @@
 #include "plugin.h"
 #include "plugin-api.h"
 #include "elf-bfd.h"
+#if !defined (HAVE_DLFCN_H) && defined (HAVE_WINDOWS_H)
+#include <Windows.h>
+#endif
 
 /* The suffix to append to the name of the real (claimed) object file
    when generating a dummy BFD to hold the IR symbols sent from the
@@ -128,6 +131,31 @@ static const enum ld_plugin_tag tv_header_tags[] =
 /* How many entries in the constant leading part of the tv array.  */
 static const size_t tv_header_size = ARRAY_SIZE (tv_header_tags);
 
+#if !defined (HAVE_DLFCN_H) && defined (HAVE_WINDOWS_H)
+
+#define RTLD_NOW 0	/* Dummy value.  */
+
+static void *
+dlopen (const char *file, int mode ATTRIBUTE_UNUSED)
+{
+  return LoadLibrary (file);
+}
+
+static void *
+dlsym (void *handle, const char *name)
+{
+  return GetProcAddress (handle, name);
+}
+
+static int
+dlclose (void *handle)
+{
+  FreeLibrary (handle);
+  return 0;
+}
+
+#endif /* !defined (HAVE_DLFCN_H) && defined (HAVE_WINDOWS_H)  */
+
 /* Helper function for exiting with error status.  */
 static int
 set_plugin_error (const char *plugin)