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.
Merge pull request #150 from rolk/150_underlink

Link issues in release/2013.03 branch on CentOS 5.9