plugin-loader: Add facility to register plugin constructors

Enabled when building monolithically and statically.

This should allow us to work around the -whole-archive issue with
libtool.  If the libraries register the plugin constructors they provide
they reference the constructors and will therefore prevent the linker from
removing these seemingly unused symbols from the final executable.

For use cases where dlsym() can be used, e.g. because the static libraries
are manually linked with -whole-archive (Linux) or -force-load (Apple),
this can be disabled by passing ss_cv_static_plugin_constructors=no to
the configure script.

src/libstrongswan/plugins/plugin_loader.h

@@ -168,4 +168,14 @@ plugin_loader_t *plugin_loader_create();
  */
 void plugin_loader_add_plugindirs(char *basedir, char *plugins);
 
+#ifdef STATIC_PLUGIN_CONSTRUCTORS
+/**
+ * Register a plugin constructor in case of static builds.
+ *
+ * @param name         name of the plugin
+ * @param constructor  constructor to register (set to NULL to unregister)
+ */
+void plugin_constructor_register(char *name, void *constructor);
+#endif
+
 #endif /** PLUGIN_LOADER_H_ @}*/