Accept CFLAGS/CPPFLAGS/CXXFLAGS when building on macOS

[alebcay]

It would be nice if we could use these variables to pass additional flags at build time, e.g. -std=c++20, similar to how can be done when building using config.linux. Or if there's a different way preferred to pass such flags, it would be helpful to know.

Apologies if this is not the best way to submit patches.