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.

Tested on CentOS 5.9.