OS X compatibility fixes

Code fixes for compatibility with OS X, see https://github.com/minetest/minetest/issues/1242. I have tested this on OS X and Linux. Some code is borrowed from github.com/constcast/vermont, which is also GPL'ed.
this should be changed the same way like https://github.com/minetest/minetest/pull/1420/files#discussion_r14264430