clean-up, compatibility improvements, add keep_system

[CL-Jeremy]

This PR:

• Adds keep_system flags as per #48
• Improves detection of options not available/known to be buggy in freedesktop.org pkg-config and older versions of pkgconf, raising an error if version requirement is not met (related: #75)
• Various code and docstring clean-ups

To do:

• Add test cases for these changes