build-osx-libs: Update GnuTLS to 3.6.13 (unbreak macOS 10.15)

This includes a workaround that should be removed when it is fixed upstream (LDFLAGS=).
It disables guile support, since we do not need it and it fails to compile too.

Patch by: Krinkle

Reviewed By: wraitii

Fixes #5729

Differential Revision: https://code.wildfiregames.com/D2716