How do I enable MPI in a CMake install?

[VictorEijkhout]

Based on the line

add_feature_info(MPI P4EST_ENABLE_MPI "MPI features of ${PROJECT_NAME}")
``
I tried both 

-D P4EST_ENABLE_MPI=ON
-D CMAKE_P4EST_ENABLE_MPI=ON

but both lead to 

-- The following features have been disabled:

• MPI, MPI features of P4EST

and the second one leads to all sorts of other mayhem.

So how *do* I enable MPI?

[VictorEijkhout]

......and the answer is obvious, once you start reading the cmake include files:

-D SC_ENABLE_MPI=ON