Avoid false positive SLEPc detection

If we seem to have SLEPc, but there's some reason we can't scarf build
configuration from SLEPc, then we really don't have SLEPc.

I got caught by this when trying a new PETSC_ARCH for which I hadn't
yet built SLEPc.
Eigen 3.2.1 source
update symlink
update derived Makefile.in
removing eigen 3.2.0