fix the test for the quadmath library

Seems like I forgot to include the "CheckCSourceCompiles" macro before
using it. Since it worked for me before, I'm wondering what made this
happen. Anyway, it should now work correctly in all cases.
cmake: fix the SuperLU test

for me, it was completely broken. (it worked fine if you did not want
to use it, though.) Now it (hopefully) follows the standard OPM
variable naming conventions, allows to use the internal SuperLU BLAS
library, and works fine if superLU is not installed on a system-wide
basis.

v2: cache the POST_2005 variable to make writing it to config.h more
    reliable (on my kubuntu 13.04 system it did not work without it)
v3: re-add a proper "post 2005" check from newer versions of the dune tests
Merge pull request #346 from andlaus/fix_superlu_and_quadmath

Fix superlu and quadmath