meta/common: Put META_EXPORT in its own header

Want to include it without including all of clutter and cogl.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/3304>

src/meta/common.h

@@ -27,6 +27,7 @@
 #include <glib.h>
 
 #include "clutter/clutter.h"
+#include "meta/meta-base.h"
 #include "meta/meta-enums.h"
 
 /**
@@ -42,8 +43,6 @@
 /* Replacement for X11 CurrentTime */
 #define META_CURRENT_TIME 0L
 
-#define META_EXPORT __attribute__((visibility("default"))) extern
-
 #define MAX_BUTTONS_PER_CORNER META_BUTTON_FUNCTION_LAST
 
 /* Keep array size in sync with MAX_BUTTONS_PER_CORNER */

src/meta/meson.build

@@ -9,6 +9,7 @@ mutter_public_headers = [
   'keybindings.h',
   'main.h',
   'meta-backend.h',
+  'meta-base.h',
   'meta-background.h',
   'meta-background-actor.h',
   'meta-background-content.h',

src/meta/meta-base.h

@@ -0,0 +1,21 @@
+/*
+ * Copyright (C) 2001 Havoc Pennington
+ * Copyright (C) 2005 Elijah Newren
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, see <http://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+#define META_EXPORT __attribute__((visibility("default"))) extern