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This crate builds math symbols from a musl checkout and provides a Rust interface. The intent is that we will be able to compare our implementations against musl on more than just linux (which are the only currently the only targets we run `*-musl` targets against for comparison). Musl libc can't compile on anything other than Linux; however, the routines in `src/math` are cross platform enough to build on MacOS and windows-gnu with only minor adjustments. We take advantage of this and build only needed files using `cc`. The build script also performs remapping (via defines) so that e.g. `cos` gets defined as `musl_cos`. This gives us more certainty that we are actually testing against the intended symbol; without it, it is easy to unknowingly link to system libraries or even Rust's `libm` itself and wind up with an ineffective test. There is also a small procedure to verify remapping worked correctly by checking symbols in object files.