cmake: Allow building ADIOS with MPI using a serial HDF5

The HDF5 IO engine now uses MPI optionally.  Support is enabled at
runtime when the application creates an ADIOS factory with a MPI
communicator.  This option is available only if HDF5 was built with MPI
support.  Therefore we can now optionally build ADIOS with MPI support
using a HDF5 *without* MPI support and adapt functionality accordingly.

Update the build system to support this combination.

cmake/DetectOptions.cmake

@@ -155,21 +155,13 @@ endif()
 # HDF5
 if(ADIOS2_USE_HDF5 STREQUAL AUTO)
   find_package(HDF5 COMPONENTS C)
-  if(HDF5_FOUND AND
-     ((ADIOS2_HAVE_MPI AND HDF5_IS_PARALLEL) OR
-      (NOT ADIOS2_HAVE_MPI AND NOT HDF5_IS_PARALLEL)))
+  if(HDF5_FOUND AND (NOT HDF5_IS_PARALLEL OR ADIOS2_HAVE_MPI))
     set(ADIOS2_HAVE_HDF5 TRUE)
   endif()
 elseif(ADIOS2_USE_HDF5)
   find_package(HDF5 REQUIRED COMPONENTS C)
-  if(ADIOS2_HAVE_MPI)
-    if(NOT HDF5_IS_PARALLEL)
-      message(FATAL_ERROR "MPI is enabled but serial HDF5 is detected.")
-    endif()
-  else()
-    if(HDF5_IS_PARALLEL)
-      message(FATAL_ERROR "MPI is disabled but parallel HDF5 is detected.")
-    endif()
+  if(HDF5_IS_PARALLEL AND NOT ADIOS2_HAVE_MPI)
+    message(FATAL_ERROR "MPI is disabled but parallel HDF5 is detected.")
   endif()
   set(ADIOS2_HAVE_HDF5 TRUE)
 endif()