Requirements for mason_compile for iOS builds

[jfirebaugh]

For creating iOS universal binaries, 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

[kkaefer]

I'm not seeing a way that would be easier, given that we need to integrate with other build systems that we don't have any control over.