FindMPI: Fix regression when compiling with MPI wrappers
The change in !4153 (merged) converted a CMake-language function to a macro. However, it contains a return()
call that now applies in the caller's scope, which is in correct. The return()
is meant to skip work when using a MPI compiler wrapper as the main compiler. Move that condition to the call site instead.
Fixes: #20234 (closed)