Fixes #483. Move to use CACHE INTERNAL

[mathomp4]

Closes #483

This PR updates the main CMake from using PARENT_SCOPE to CACHE INTERNAL. The former seemed to have issues when pFUnit was used via FetchContent from a subdirectory. The latter is a bit of a sledgehammer, but makes sure variables are defined in all scopes.

We also update a few bits of CMake to avoid some warnings:

• Change some cmake_minimum_required to 3.12
• Fix some COMMENT as CMake expects a single argument