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.
On Linux all libraries that are specified on the command-line will be
referred to in the resulting binary. There may be find modules which
just adds everything to satisfy every possible dependency; we want to
discard those unnecessary libraries.
