If we have checked out a newer Eigen3 directory in a sibling directory
to ours, assume that this is because the system version is obsolete and
that we want to use this one instead.
Eigen3 is a template-library, so we must compile the source code
directly together with ours instead of linking to a library.
If the build script gives us the location of a "built" Eigen3,
meaning a directory in which CMake has been run, locate the true
source directory from the cache entries.
By setting EIGEN3_ROOT, we can point directly to the path of the
library checkout we want to use. No other searching will then take
place.
Notice that if you have found an Eigen3 installation in a previous
configure, it will not find a new version even if you introduce the
EIGEN3_ROOT variable!
