support configure summaries

typical project configuration output could look like this:

hardware drivers:
  anton ... yes
  berta ... no
  eumel ... maybe
knolf ..... modular (very)
libbar .... no
zoo ....... yes

Note: libbar was not found, which will cause wurst functionality to be not available.
libbar home page: http://libbar.deadmeat.com

the sources for these messages may be arbitrary products and modules, so this must scale up when cascading projects (and conversely, still work when only subprojects are built).
this means that there needs to be a central "ouput collection facility". this may be implemented via QBS-262, and may hook into QBS-60 to get activated when all configure tests have been processed.
the output itself needs to be tagged, so it can be automatically put into sections, hierarchies, and be sorted.