Use `cmake` crate to build SFML via CMake

[ChrisThrasher]

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https://crates.io/crates/cmake

There is a CMake crate that can be used to build CMake-based libraries. This would allow for removing all CSFML source code from the repo and will make future updates easier as this repo will no longer have to maintain an in-source fork of the library. It also opens up the possibility of building SFML from source in the build.rs file so that users aren't forced to do that themselves.

[crumblingstatue]

The vendored CSFML is a custom fork tailored for rust-sfml, so there is no intention of removing it.

However, building SFML with CMake might be something worth exploring.

[crumblingstatue]

We now have a vendored-sfml branch, for people curious about vendoring and statically linking SFML.