cmake: Improve safety and robustness during building `crc32c` subtree

It was agreed not to adopt the upstream buildsystem the crc32c subtree.

So this PR ensures that our custom build script properly handles potentially harmful situations, including:

1. The upstream buildsystem changes remaining unnoticed during the subtree update, even when some of those changes need to be reflected in crc32c.cmake.

2. The order of include(cmake/introspection.cmake) and include(cmake/crc32c.cmake) being inadvertently altered during refactoring or other changes in the main buildsystem.