fix the GCC 4.6 stuff in a more backward compatible way and fix a lot of "variable set but not used" warnings on GCC 4.6

if the compiler doesn't know about constexpr

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@5676 2fb0f335-1f38-0410-981e-8018bf24f1b0