Sync common files (CMakeLists.txt, Catch2) with main repo

This repo has its own set of common support files. it would be worthwhile to have them synced with [exericm/cpp](https://github.com/exercism/cpp)

See also https://github.com/exercism/cpp/issues/1029

Compare this repository's CMakeLists.txt and Catch2 support files with the corresponding common files in the exercism/cpp repository. Read exercism/cpp issue 1029 for the expected scope, then synchronize the files and verify that the test runner still builds and runs its tests.

索引モデルが issue の本文から書いたものです。