Requirements for mason_compile for iOS builds

Nobody has claimed this yet.

[For creating iOS universal binaries](https://github.com/mapbox/mason/blob/432ba70495e6651caacfade4e9288d58135c3498/mason.sh#L414-L447), `mason.sh` requires some things from the `script.sh`'s `mason_compile` that aren't obvious:
- `mason_compile` (or the commands it delegates to) must place static archives in `${MASON_PREFIX}/lib`.
- `mason_compile` (or the commands it delegates to) must use the `CFLAGS` environment variable (even if also using `CXXFLAGS`).
- `mason_compile` (or the commands it delegates to) must support being called multiple times, and must do a clean rebuild when called with `CFLAGS` different from the previous call. In particular if `mason_compile` delegates to `make` commands, they need to have way of ensuring a rebuild outside of normal file modification times.

This ticket is to discuss if there is some way to reduce this coupling between `mason.sh` and `mason_compile`, or where/how this should be documented if not.

cc @kkaefer @springmeyer

Start with mason.sh lines 414-447 and inspect the script.sh mason_compile entry point. Trace how static archives, CFLAGS, repeated calls, and clean rebuilds are handled, then decide whether the coupling should be reduced or documented. Done means the iOS build requirements are either decoupled or clearly documented.

Written by the indexing model from the issue text.