Taking libmesh-specific comment out of eigen.m4

I'm planning to swipe a copy of this for some other projects and I'd
like to keep it in sync.
Bugfix for enableeigen='' case
Remember to re-bootstrap...
Make sure we're using C++ to test for Eigen
initial infrastructure to support radial basis functions as an alternative MeshfreeInterpolation scheme
Wendland Radial Basis Function
beginnings of Radial Basis Interpolation implementation