ENH: support alternative build-root location (#2286)

- specify any of these

    ./Allwmake -build-root=...
    wmake -build-root=...
    FOAM_BUILDROOT=... wmake

  these specify an alternative root where build artifacts are to land.
  Currently only used as an alternative for the 'build/' hierarchy
  since the 'platforms/' target normally includes inputs as well.

  Possible use:
  ```
  (
      export WM_MPLIB="%{foam_mplib}"
      export FOAM_MPI="%{foam_mpi}"
      export MPI_ARCH_PATH="%{mpi_prefix}"

      export FOAM_BUILDROOT=/tmp/mpibuild
      export FOAM_MPI_LIBBIN="$FOAM_BUILDROOT/platforms/$WM_OPTIONS/lib/$FOAM_MPI"

      src/Pstream/Allwmake-mpi
   )
   ```