memfd: add workaround for systems without memfd

Just a quick workaround to use a tmpfile if memfd is not available. We
should try to ensure that we get something on tmpfs, but this is probably
good enough for a fallback.

lib/sp-platform.h

@@ -0,0 +1,30 @@
+/* sp-platform.h
+ *
+ * Copyright (C) 2016 Christian Hergert <christian@hergert.me>
+ *
+ * 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 3 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/>.
+ */
+
+#ifndef SP_PLATFORM_H
+#define SP_PLATFORM_H
+
+#include <glib.h>
+
+G_BEGIN_DECLS
+
+int sp_memfd_create (const gchar *desc);
+
+G_END_DECLS
+
+#endif /* SP_PLATFORM_H */