cmake: Forbid in-source builds

[hebasto]

In-source builds with CMake are incompatible with Autotools because the Makefile files are being overwritten.

This PR provides a clear error message if the user attempts to perform an in-source build.

Unfortunately, this approach does not prevent the source directory from being polluted with a few build artifacts. For more details, please see:

• https://discourse.cmake.org/t/preventing-in-source-builds/881
• https://gitlab.kitware.com/cmake/cmake/-/issues/6672

Resolves https://github.com/hebasto/bitcoin/issues/306.

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The allowance of in-source builds can be reconsidered after completing the migration from Autotools.

[hebasto]

Unfortunately, this approach does not prevent the source directory from being polluted with a few build artifacts.

If in a git repository, the git clean -fd command will help delete these artifacts.