Skip the writable test if the file is not writable

Since make distcheck operates on a readonly source tree.

glib/tests/mappedfile.c

@@ -1,5 +1,9 @@
+#include <config.h>
 #include <glib.h>
 #include <string.h>
+#ifdef HAVE_UNISTD_H
+#include <unistd.h>
+#endif
 
 static void
 test_empty (void)
@@ -38,6 +42,12 @@ test_writable (void)
   const gchar *old = "MMMMMMMMMMMMMMMMMMMMMMMMM";
   const gchar *new = "abcdefghijklmnopqrstuvxyz";
 
+  if (access (SRCDIR "/4096-random-bytes", W_OK) != 0)
+    {
+      g_test_message ("Skipping writable mapping test");
+      return;
+    }
+
   error = NULL;
   file = g_mapped_file_new (SRCDIR "/4096-random-bytes", TRUE, &error);
   g_assert_no_error (error);