Modules may be skipped for different reasons: SKIP_MODULES
is the traditional approach to suppress some, but other modules
may have unmet build requirements (e.g. Plasma Look-and-Feel,
or the Partitioning module) and should be able to opt-out
of being built. For all those skipped, log it explicitly after
all the modules have been examined.
Only CMake-based (e.g. C++) modules support opting-out in this way.
