exchangers: Make send mode feature optional.

configure.ac

src/xt_config.c

src/xt_exchanger_irecv_isend.c

src/xt_exchanger_irecv_isend_ddt_packed.c

src/xt_exchanger_irecv_isend_packed.c

src/xt_exchanger_irecv_send.c

src/xt_exchanger_mix_isend_irecv.c

tests/test_exchanger_parallel.c

tests/test_exchanger_parallel_f.f90