Blender Git Loki

Bump minimum CMake version to 3.10

This bumps the minimally required CMake version from 3.5 to 3.10, as
discussed in D8405.

Since D7649 landed, there has been the `gtest_discover_tests()` call to
discover individual unit tests in `bin/tests/blender_test`. This
function was introduced in CMake 3.10.

Since there were no complaints about this incompatibility, I suspect
that a newer version is already in use by the majority of the
Blender-building people.