[Distutils] recommended strategy for missing development tools

[Robin Becker]

A reportlab user says his pip install fails to create an importable C extension. 
He said

"The platform is Mac OS X 10.11.6, aka "El Capitan". Pip didn't complain"

which is probably true since the extension is not required for reportlab's main 
usage.

I tried to reproduce on OS X 10.10.5, but my machine has xcode installed and the 
extension was correctly produced.

I don't have any code in setup.py to prevent compilation; is there a way to 
alert users to the non-build of the extension(s).
-- 
Robin Becker