FindMPI: Choose definitions for MPI_CXX_SKIP_MPICXX before first use

Fixes: #22391

Modules/FindMPI.cmake

@@ -1443,6 +1443,7 @@ foreach(LANG IN ITEMS C CXX Fortran)
       set(MPI_CXX_SKIP_MPICXX FALSE CACHE BOOL "If true, the MPI-2 C++ bindings are disabled using definitions.")
       mark_as_advanced(MPI_CXX_SKIP_MPICXX)
     endif()
+    _MPI_adjust_compile_definitions(${LANG})
     if(NOT (MPI_${LANG}_LIB_NAMES AND (MPI_${LANG}_INCLUDE_PATH OR MPI_${LANG}_INCLUDE_DIRS OR MPI_${LANG}_COMPILER_INCLUDE_DIRS)))
       set(MPI_${LANG}_TRIED_IMPLICIT FALSE)
       set(MPI_${LANG}_WORKS_IMPLICIT FALSE)
@@ -1547,7 +1548,6 @@ foreach(LANG IN ITEMS C CXX Fortran)
     endif()
     _MPI_assemble_libraries(${LANG})
 
-    _MPI_adjust_compile_definitions(${LANG})
     # We always create imported targets even if they're empty
     _MPI_create_imported_target(${LANG})