BUG: SuperLU: fix missing declaration of dlamch

SuperLU 4.3 called some functions returning floats without declaring
them. On i386@linux call convention, this fails to clear floating point
registers after call. As a result, NaN can appear in the next floating
point operation made.
Merge pull request #3388 from pv/superlu-bug

BUG: SuperLU: fix missing declaration of dlamch