add CMake build system

[Be-ing]

I am packaging twolame in vcpkg for Tenacity and do not want to deal with Visual Studio project files.

[Be-ing]

The GitHub Actions runs won't show up on pull requests for this repo until this is merged. In the meantime you can see the results on my fork: https://github.com/Be-ing/twolame/actions

[Be-ing]

ping

[generic-pers0n]

I’ll give this a bump to @njh even though this PR has sat inactive for more than 2 years.

I’ll be quite honest here: I’m a little eager, perhaps too eager, to see this PR merged. Particularly, I want to upstream an existing TwoLAME port that currently uses this branch, and I prefer to use upstream sources wherever possible. I also believe there are other benefits to using CMake over Autotools as a build system, particularly Ninja for faster builds and modern versions of Visual Studio. In the case of the latter, you wouldn’t need to maintain the ancient Visual Studio Project files anymore and let them be.