cmake: a little helper function

for notifying the user about 3rdparty dependencies, which get pulled in
but will not get installed together with PFASST++
cmake: use provided feature summary (not custom one)
cmake: fix, cleanup and improve 3rdparty configuration

this shoud fix #124
cmake: remove dead code

did I mention, that this dirty hack and fiddling with internal properties
has been obsolete and removed? :+1:
cmake: increase timeout for 3rdparty downloads

also some additional cleanup
Merge pull request #126 from torbjoernk/feature/fix-cmake

fixing #124 and some improvements to 3rdparty configuration