BEMAN_USE_MODULES should probably default to OFF

[pdimov]

Currently, BEMAN_USE_MODULES is automatically set to ON when module support is detected. This is usually not what I would want, and I suspect is not what other users would want, as module support is still unreliable (and in this case, doesn't buy much.)

It would probably be better if BEMAN_USE_MODULES unconditionally defaults to OFF, making module support opt-in.

This issue is made more pressing by #246, because currently consumers need to care about BEMAN_USE_MODULES in order to know to what target to link.

[ednolan]

Agreed, I'll look into this