If Python isn't found, or PythonQt isn't found, there is no point
in copying over the modules into the build tree; this may even be
misleading because the files are there, but won't work and this
will be noticed at runtime only.
Instead, skip the modules and explain why.
Make a function out of explaining-skipped-modules, and call it
not only after collecting all the modules, but also after
the feature summary, so that it's quite clear which modules
are skipped.
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.
