configure: report version of compiler as given by --version
configure: report version of compiler as given by --version

Previously, the -dumpversion option was used to get a more concise
version string, but this doesn't actually match with the actual version
reported by --version on clang. Go figure... In any case, I reckon it's
probably safest to report the more 'official' --version information.

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3