Ensure C language when requiring MPI

[dg0yt]

With PARALLEL enabled, the exported targets depend on MPI::MPI_C. However, this target isn't created by find_package(MPI) when the consuming project has only language CXX enabled. There is also no good diagnostics because MPI is found successfully.

(Alternative: Use find_package(MPI ... COMPONENTS C). This would enable proper diagnostics. But simply enabling language C is more effective. This might be reconsidered when fully supporting cmake config search by switching to find_dependency() instead of find_package(... REQUIRED).)

Noticed while testing vcpkg port gdal with hdf5 being a transitive dependency - the minimal test port had just CXX enabled.

This PR: Build successful.

Before this PR:

CMake Error at /Users/vcpkg/Data/installed/arm64-osx/share/hdf5/hdf5-targets.cmake:61 (set_target_properties):
  The link interface of target "hdf5::hdf5-static" contains:

    MPI::MPI_C

  but the target was not found.  Possible reasons include:

    * There is a typo in the target name.
    * A find_package call is missing for an IMPORTED target.
    * An ALIAS target is missing.

Diagnostics-only alternative:

CMake Error at /<redacted>/vcpkg/vcpkg/downloads/tools/cmake-3.30.1-linux/cmake-3.30.1-linux-x86_64/share/cmake-3.30/Modules/FindPackageHandleStandardArgs.cmake:233 (message):
  Could NOT find MPI (missing: MPI_C_FOUND C)

      Reason given by package: MPI component 'C' was requested, but language C is not enabled.  

[byrnHDF]

While I think this is just fine - the only worry is if there will be any unintended issues. I Can't think of any since the library is designed to be built with "C".

[dg0yt]

I acknowledge that there are some restrictions on where enable_language can be used:
https://cmake.org/cmake/help/latest/command/enable_language.html
I believe that these cases should be rare for normal use of find_package. But OTOH I have seen enough weird cmake code...