Use CMake's CMP0077 policy if available

[thelink2012]

Description

Using the project as an add_subdirectory (and setting options with set before doing so) produces an warning regarding policy CMP0077.

This happens because the behavior of using set on variables which will be cached (by option) has changed in CMake 3.12. Before, the value set would be unset, and now the value set is honored.

The project should have CMP0077 set to NEW to ensure it understands this change (the old behaviour is deprecated and may be removed in the future). Doing this outside doctest tree (i.e. the tree that performs add_subdirectory on it) has no effect.

GitHub Issues

Didn't find it necessary. The change is minimal and any necessary discussion can take place in the PR.

[onqtam]

Thanks!