What you really want, after configure has confirmed openmpi with the macro check, is to extract the libraries listed in the output of mpif90 -v test.f that are needed. Ideally someone could update ompi_config to output the link flags for 3 cases: C++ linking without using mpicxx F90 linking without using mpif90 F90 linking without f90 but that's a nontrivial bit of work.
The fortran lib extraction process can be rather hairy if done portably-- the babel team at llnl has largely solved it in babel-$VERSION/runtime/m4/llnl_confirm_babel_f90_support.m4 and related files obtainable from http://www.llnl.gov/CASC/components/docs/babel-1.1.0.tar.gz Ben
