Rename option ENABLE_{STATIC,SHARED} into BUILD_{STATIC,SHARED}

Thanks to Pat Marion for suggesting the change.

Doing so will prevent the option from falling into the long list of
ENABLE_<module> variables.

The options is now displayed at the beginning of the list.
Merge branch 'rename-enable-static-shared-option' into next

* rename-enable-static-shared-option:
  Rename option ENABLE_{STATIC,SHARED} into BUILD_{STATIC,SHARED}