apr 1.5.1 (new formula)

New formulae for apr, apr-util and a reworded/repurposed requirement to fit both. The existing syntax in-formulae of ` depends_on :apr ` doesn't need changing, It should and appears to work as is. *(Which is nice, usually when I expect something to work as is it doesn't).*
Please move this conditional inside the `env` block.

It may seem silly, but `MacOS::CLT.installed?` shells out, and shelling out as a side-effect of requiring this file has a load-time performance penalty.