macOS packaging is broken on CMake 3.13

Here's a Travis build demonstrating the issue: https://travis-ci.org/OpenMW/openmw/jobs/472133790#L1949.

Here's CMake bug I've filed: https://gitlab.kitware.com/cmake/cmake/issues/18726.

Temporary solution: use CMake 3.12 for packaging.

UPD workarounds:

• https://github.com/OpenMW/openmw/pull/2101
• https://github.com/OpenMW/openmw/pull/2102