non-free properties in usage requirements don't work, generally

[grafikrobot@…]

One effect of non-free props not "working" is that adding them to a usage requirement, and a counterpart to a variant, merges the flags for both settings into the build commands. Two options possible to fix it:

• Make it work, somehow :-)
• Or, generate a warning (or error) when one has non-free props in usage requirements.