shell-utils: Add missing macros and C++ compat/install header

Allow C++ access from within C code and add macros to avoid
multiple inclusion of the header. Also, install the header for other
users of libweston to actually make use of it.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>

include/libweston/meson.build

@@ -9,6 +9,7 @@ install_headers(
 	'zalloc.h',
 	'remoting-plugin.h',
 	'pipewire-plugin.h',
+	'shell-utils.h',
 	subdir: dir_include_libweston_install
 )
 

include/libweston/shell-utils.h

@@ -1,5 +1,5 @@
 /*
- * Copyright 2021 Collabora, Ltd.
+ * Copyright 2021, 2022 Collabora, Ltd.
  *
  * Permission is hereby granted, free of charge, to any person obtaining a
  * copy of this software and associated documentation files (the "Software"),
@@ -21,6 +21,7 @@
  * DEALINGS IN THE SOFTWARE.
  *
  */
+#pragma once
 
 #include "config.h"
 #include "shared/helpers.h"
@@ -29,6 +30,10 @@
 #include <libweston/config-parser.h>
 #include <string.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 /* parameter for weston_curtain_create() */
 struct weston_curtain_params {
 	int (*get_label)(struct weston_surface *es, char *buf, size_t len);
@@ -72,3 +77,7 @@ weston_shell_utils_curtain_destroy(struct weston_curtain *curtain);
 
 uint32_t
 weston_shell_get_binding_modifier(struct weston_config *config, uint32_t default_mod);
+
+#ifdef __cplusplus
+}
+#endif