Protect against underlinking of UMFPACK library

If libumfpack.so does not declare an explicit dependency on libamd.so,
the linker option -Wl,--as-needed will cause an undefined reference
since we don't use libamd.so ourself. If the configuration determines
that this may be the case, a linker option that forces linking the the
AMD library is added for the SuiteSparse library, and the general
configuration then gets out of the way if this is set.