Use keyword "extern" on windows too, and hope to fix build warnings.
Concerned warnings are, for example:
./src/include/smpi/smpi_interface.h:29:43: warning: array mpi_coll_gather_description assumed to have one element [enabled by default]
./src/include/smpi/smpi_interface.h:37:43: warning: array mpi_coll_allgather_description assumed to have one element [enabled by default]
./src/include/smpi/smpi_interface.h:45:43: warning: array mpi_coll_allgatherv_description assumed to have one element [enabled by default]
./src/include/smpi/smpi_interface.h:54:43: warning: array mpi_coll_allreduce_description assumed to have one element [enabled by default]
./src/include/smpi/smpi_interface.h:63:43: warning: array mpi_coll_alltoall_description assumed to have one element [enabled by default]
./src/include/smpi/smpi_interface.h:71:43: warning: array mpi_coll_alltoallv_description assumed to have one element [enabled by default]
./src/include/smpi/smpi_interface.h:80:43: warning: array mpi_coll_bcast_description assumed to have one element [enabled by default]
./src/include/smpi/smpi_interface.h:89:43: warning: array mpi_coll_reduce_description assumed to have one element [enabled by default]
./src/include/smpi/smpi_interface.h:97:43: warning: array mpi_coll_reduce_scatter_description assumed to have one element [enabled by default]
./src/include/smpi/smpi_interface.h:105:43: warning: array mpi_coll_scatter_description assumed to have one element [enabled by default]
./src/include/smpi/smpi_interface.h:114:43: warning: array mpi_coll_barrier_description assumed to have one element [enabled by default]
