Fix some issues with GSL for `make auxchecks'

Fix some issues with GSL for `make auxchecks'

When trying to reproduce Bug 423908, I ran into some trouble performing
`make auxchecks', due to problems in GSL:

Newer compilers complain about incompatible pointer types in argument
passing between

  unsigned int *

and

  size_t *

This affects the fifth argument of function gsl_eigen_jacobi() in
eigen/jacobi.c.  Fix this by passing the right pointer type at invocation.

Also, the configure checks for IEEE comparisons and for IEEE denormalized
values don't work as intended, because they call exit() without declaring
it and thus fail independently from those features being supported.  Fix
this by using `return' instead.