Fixes GnuTLS build on macOS.
Fixes macOS linker warning "PIE disabled absolute - addressing not allowed".

Updates nettle to 3.5.1, GnuTLS to 3.6.8, gloox to 1.0.22.
Disables TCP fast open feature of GnuTLS (requires 10.11, no SDK build support).
Fixes GnuTLS detection of GMP by adding it to LIBS flag.
Disables getaddrinfo on gloox 1.0.22. Lobby connections failed during server hostname resolution.
Adds --with-pic to GMP build to force consistent PIC usage.
Adds -N flag to patch commands to avoid reapplying them.
Removes unneeded build flags.
Documents --enable-fat configure flag: GMP and nettle detect CPU-specific features, fat binaries let us build and run them on different CPUs (see D1772).

Fixes #5453, 5489. Refs #5481.
Tested by: kali0ad, trompetin17
Reviewed by: elexis, trompetin17

Differential Revision: ​https://code.wildfiregames.com/D2057