ENH: add global properties for collecting enabled/disabled features during
the cmake run and add macros print_enabled/disabled_features() and
set_feature_info(), so projects can get a nice overview at the end of the
cmake run what has been found and what hasn't
FIND_PACKAGE() automatically adds the packages to these global properties,
except when used with QUIET
Maybe this can also be useful for packagers to find out dependencies of
projects.

Alex