CMake config depends on MPI but does not find it

[svenevs]

Describe the bug
The hdf5 library code installed had #include statments looking for MPI that could not be found.

Redirected CC @jhendersonHDF if interested.

hpc-io/vfd-gds#8 (comment)

If I compile hdf5 +mpi (specifically 1.13.2 via spack with OpenMPI), my consuming cmake code should only need to find_package(HDF5 CONFIG REQUIRED) and I link against hdf5-static|shared. I should not have to find_package(MPI REQUIRED) and link against it in addition.

Expected behavior
If mpi is a dependency of hdf5, hdf5's CMake should find that dependency.

Platform (please complete the following information)

• HDF5 version (if building from a maintenance branch, please include the commit hash): 1.13.2
• OS and version: multiple linux
• Compiler and version: multiple gcc
• Build system (e.g. CMake, Autotools) and version: spack
• Any configure options you specified: I do not have access to these anymore.
• MPI library and version (parallel HDF5): OpenMPI

[lrknox]

followed link hpc-io/vfd-gds#8 (comment) near the top, which in later comments implies it was resolved. Also ran "spack install hdf5-vfd-gds" which seemed to be the package that reported the problem. It built with no issues.