Error on undefined variables

[springmeyer]

I'm surprised we've been living without this. Undefined variables can cause major headaches in package scripts.sh. I just hit this with a cp -r ${VARIABLE}/ ${MASON_PREFIX}/bin where VARIABLE was undefined so mason attempted to copy my whole user folder. #neveragain

[springmeyer]

@brunoabinader - can you kick the tires on this and ensure the cross compiles you maintain still work? I had to make minor changes to MASON_PLATFORM_VERSION variable handling to avoid errors with this - would be good to confirm it still works. If you do find breakages then it will be a sign we are missing unit test coverage.

[springmeyer]

This caught that ${MASON_SDK_PATH} is undefined in the zlib system package at https://github.com/mapbox/mason/blob/mapnik-3.0.13/scripts/zlib/system/script.sh#L26