Allow static linking for SuiteSparse to be forced

If -DSUITESPARSE_USE_STATIC=ON, then the build system will only look for
static versions of the libraries that are part of SuiteSparse, even if
dynamic/shared versions are present on the system. Thus, the default of
preferring dynamic libraries can be overridden.

SuiteSparse is rarely built ourselves, but still uncommon enough to not
be present on computing clusters.

This patch allows us to install the libraries on a workstation, for
instance from package suitesparse-devel and link to it statically
without having to maintain our own build tree.
Merge pull request #410 from rolk/410_static

Allow static linking for SuiteSparse to be forced