Skip Fortran unittests if no Fortran MPI support is found

unittest/fortran/CMakeLists.txt

@@ -10,6 +10,11 @@ if(NOT CMAKE_Fortran_COMPILER_ID)
   message(STATUS "Skipping Tests for the LAMMPS Fortran Module: cannot identify Fortran compiler")
   return()
 endif()
+find_package(MPI QUIET)
+if(BUILD_MPI AND NOT MPI_Fortran)
+  message(STATUS "Skipping Tests for the LAMMPS Fortran Module: no MPI support for Fortran")
+  return()
+endif()
 
 if(CMAKE_Fortran_COMPILER)
   enable_language(C)