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)