Windows port: compute GOBJECT_INTROSPECTION_LIBDIR at runtime.

Otherwise, we fail to properly locate the typelibs, because on Windows
the value of GOBJECT_INTROSPECTION_LIBDIR depends on where Glib has been
installed. Due to the nature of how we handle software that depends on
Glib on Windows (it is recommended that each program bundles it's private
copy), we're working in a "multi-prefixed" environment. Hence the value
computed at build time will most likely not even exist at runtime.

https://bugzilla.gnome.org/show_bug.cgi?id=620566

girepository.c

@@ -50,6 +50,39 @@ struct _GIRepositoryPrivate
 
 G_DEFINE_TYPE (GIRepository, g_irepository, G_TYPE_OBJECT);
 
+#ifdef G_PLATFORM_WIN32
+
+#include <windows.h>
+
+static HMODULE girepository_dll = NULL;
+
+#ifdef DLL_EXPORT
+
+BOOL WINAPI
+DllMain (HINSTANCE hinstDLL,
+	 DWORD     fdwReason,
+	 LPVOID    lpvReserved)
+{
+  if (fdwReason == DLL_PROCESS_ATTACH)
+      girepository_dll = hinstDLL;
+
+  return TRUE;
+}
+
+#endif
+
+#undef GOBJECT_INTROSPECTION_LIBDIR
+
+/* GOBJECT_INTROSPECTION_LIBDIR is used only in code called just once,
+ * so no problem leaking this
+ */
+#define GOBJECT_INTROSPECTION_LIBDIR \
+  g_build_filename (g_win32_get_package_installation_directory_of_module (girepository_dll), \
+		    "lib", \
+		    NULL)
+
+#endif
+
 static void
 g_irepository_init (GIRepository *repository)
 {