Handle MFEM's CMake build system better

[white238]

If MFEM is built using their CMake build system, Axom does not export or recreate the axom::mfem target for downstream users.

Serac does this but I forgot to move this back to Axom:

https://github.com/LLNL/serac/blob/6b8c563a09eec70ba424b684921edac80158258d/cmake/serac-config.cmake.in#L78-L89 https://github.com/LLNL/serac/blob/6b8c563a09eec70ba424b684921edac80158258d/cmake/thirdparty/FindMFEM.cmake#L30