MacOS compilation fix

The “Fix compilation on OS X for RTLD_DEEPBIND.” commit is a cherry-pick from https://github.com/jdolan/NetRadiant by @jdolan, see ce3a086.

cu-kai reported this old fork build on MacOS but not the upstream NetRadiant master, so I imported that fix hoping it's enough.

I have no way to test this, so testing from mac users would be very helpful.