[gtk+/portal] Better sandbox detection

[Matthias Clasen <matthiasc src gnome org>]

commit e0ff42761608adc78ef3bbeec499790ccac86b90
Author: Matthias Clasen <mclasen redhat com>
Date:   Wed Jul 6 23:23:09 2016 -0400

    Better sandbox detection
    
    Use g_build_filename, and cache the result.

 gtk/gtkprivate.c |   26 +++++++++++++++-----------
 1 files changed, 15 insertions(+), 11 deletions(-)
---
diff --git a/gtk/gtkprivate.c b/gtk/gtkprivate.c
index 7f2a335..02448d7 100644
--- a/gtk/gtkprivate.c
+++ b/gtk/gtkprivate.c
@@ -271,19 +271,23 @@ _gtk_ensure_resources (void)
 gboolean
 gtk_should_use_portal (void)
 {
-  const char *use_portal;
-  char *path;
+  static const char *use_portal = NULL;
 
-  path = g_strdup_printf ("%s/flatpak-info", g_get_user_runtime_dir ());
-  if (g_file_test (path, G_FILE_TEST_EXISTS))
-    use_portal = "1";
-  else
+  if (G_UNLIKELY (use_portal == NULL))
     {
-      use_portal = g_getenv ("GTK_USE_PORTAL");
-      if (!use_portal)
-        use_portal = "";
+      char *path;
+
+      path = g_build_filename (g_get_user_runtime_dir (), "flatpak-info", NULL);
+      if (g_file_test (path, G_FILE_TEST_EXISTS))
+        use_portal = "1";
+      else
+        {
+          use_portal = g_getenv ("GTK_USE_PORTAL");
+          if (!use_portal)
+            use_portal = "";
+        }
+      g_free (path);
     }
-  g_free (path);
 
-  return g_str_equal (use_portal, "1");
+  return use_portal[0] == '1';
 }