CMake: robust version extraction for P4Est

Make the version extraction in P4Est more robust: We should not rely on
the version string to define SUBMINOR and PATCH - this doesn't have to
be the case. So, always set P4EST_VERSION_SUBMINOR and
P4EST_VERSION_PATCH to "0" in FindP4EST.cmake if no valid number could
be extracted.
Add some description of when the last two IFs can happen in version
number recognition.
Merge pull request #17 from bangerth/fix_p4est_configuration

Fix p4est configuration