Minor cleanup for conduit and hydrogen (#17136)

* Fix how the Conduit detects that the MPI compiler is the same as the
CC compiler and is more careful when it sets the MPI compilers to be
the Cray PE system compilers.

* Remove unnecessary push of the MPI compilers to the C compilers for Hydrogen.

var/spack/repos/builtin/packages/conduit/package.py

@@ -469,7 +469,7 @@ def create_host_config(self, spec, prefix, py_site_pkgs_dir=None):
             # use those for mpi wrappers, b/c  spec['mpi'].mpicxx
             # etc make return the spack compiler wrappers
             # which can trip up mpi detection in CMake 3.14
-            if cpp_compiler == "CC":
+            if spec['mpi'].mpicc == spack_cc:
                 mpicc_path = "cc"
                 mpicxx_path = "CC"
                 mpifc_path = "ftn"

var/spack/repos/builtin/packages/hydrogen/package.py

@@ -117,9 +117,6 @@ def cmake_args(self):
 
         args = [
             '-DCMAKE_INSTALL_MESSAGE:STRING=LAZY',
-            '-DCMAKE_C_COMPILER=%s' % spec['mpi'].mpicc,
-            '-DCMAKE_CXX_COMPILER=%s' % spec['mpi'].mpicxx,
-            '-DCMAKE_Fortran_COMPILER=%s' % spec['mpi'].mpifc,
             '-DBUILD_SHARED_LIBS:BOOL=%s'      % ('+shared' in spec),
             '-DHydrogen_ENABLE_OPENMP:BOOL=%s'       % ('+hybrid' in spec),
             '-DHydrogen_ENABLE_QUADMATH:BOOL=%s'     % ('+quad' in spec),