WIP: Use findMPI earlier.

Author: vehre

CMakeLists.txt

@@ -282,7 +282,9 @@ if (C_COMPILER_NAME MATCHES "^[mM][pP][iI]")
   set (MPI_C_COMPILER "${CMAKE_C_COMPILER}")
 endif()
 
-if(WIN32) # Only support building with GCC & GFortran using Intel MPI (OneAPI)
+find_package(MPI COMPONENTS C)
+
+if(WIN32 AND NOT MPI_FOUND) # Only support building with GCC & GFortran using Intel MPI (OneAPI)
   # Here we assume Intel ONEAPI and the environment is loaded
   set( MPI_ASSUME_NO_BUILTIN_MPI TRUE )
   set( MPI_CXX_SKIP_MPICXX TRUE )