el1p2c and el2p: do not require SuperLU to compile. Give a warning for el2p if...

el1p2c and el2p: do not require SuperLU to compile. Give a warning for el2p if SuperLU is not found.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@11523 2fb0f335-1f38-0410-981e-8018bf24f1b0