Change to public CMake link for MPI breaks spack builds for certain packages

[lrknox]

PR #2198 "Correct the CMake link for MPI" changed the dependency on MPi:MPI_C from PRIVATE to PUBLIC for target_link_libraries (${HDF5_LIBSH_TARGET} to resolve issues with some dependent software packages. See also #2176 "CMake config depends on MPI but does not find it"

The change introduced this error when configuring 2 or more Spack packages in E4S tests:

1 error found in build log:
25 -- Found ZLIB: /scr/lrknox/spack_lrknox/opt/spack/linux-centos7-haswell/g
cc-10.2.0/zlib-1.2.13-lik6tppfkychie657bb37cn4nxt2sewa/lib/libz.so (found
version "1.2.13")
26 -- Found OpenMP_C: -fopenmp (found version "4.5")
27 -- Found OpenMP_CXX: -fopenmp (found version "4.5")
28 -- Found OpenMP: TRUE (found version "4.5")
29 -- Found HDF5: hdf5-shared (found version "1.14.0")
30 -- Configuring done

31 CMake Error at /scr/lrknox/spack_lrknox/opt/spack/linux-centos7-haswell/g
cc-10.2.0/hdf5-1.14.0-rfvrzau53dmibmjws7rz4tinc4xebz3k/cmake/hdf5-targets
.cmake:68 (set_target_properties):
32 The link interface of target "hdf5-shared" contains:
33
34 MPI::MPI_C
35
36 but the target was not found. Possible reasons include:
37

Note that the default configuration for the hdf5 spack pacakge includes +mpi, but the packages that fail with the change may not use mpi.

[derobins]

Everyone insists that this is fixed. Closing.