Augments the extension for building C/C++ bindings with tests for SDKROOT and MACOSX_DEPLOYMENT_TARGET

Augments the extension for building C/C++ bindings with tests for SDKROOT and MACOSX_DEPLOYMENT_TARGET environment variables.

This partially addresses shortcomings in the setup of building C/C++ extensions (extensively discussed at bob.devtools#24 (closed)) by checking such environment variables before C/C++ modules and libraries can be build on macOS systems. An standard EnvironmentError is raised in case these environment variables are not properly set.

Documentation was added explaining how to properly setup the environment on macOS.