Inferring prefix for pc files

[lunacd]

Options:

• Synthesize a cps_path like this for pc files

  cps-config/tests/cps-files/lib/cps/cps-path-set.cps.in

  Line 4 in 607c449

  "cps_path": "@prefix@/${libdir}/cps/",

• Don't calculate prefix and rely solely on the PREFIX variable provided in pc files
• Treat PC file PREFIX specially and just take that as prefix

See initial discussion in #94 (comment)_

[dcbaker]

With the changes to prefix in 0.13 that add a prefix field and don't require the calculation, I think we're good? pkg-config compat can just use the prefix from the pkg-config file (if set) as if it's the same as prefix from cps.

What to do for pkg-config files that don't set a prefix (I run into them from time to time) is a bigger question.