Fix -Wcast-qual warnings from recent clang.

configure.ac

m4/acx_mpi_const_args.m4

src/Makefile.am

src/core/core_cf.c

src/xt/xt_redist.h

src/xt_config.c

src/xt_exchanger.h

src/xt_exchanger_mix_isend_irecv.c

src/xt_idxsection.c

src/xt_idxstripes.c

src/xt_idxvec.c

src/xt_mpi_ddt_cache.c

src/xt_mpi_ddt_cache.h

src/xt_mpi_ddt_wrap.h

src/xt_mpi_stripe_parse_func.h

src/xt_redist.c

src/xt_redist_collection.c

src/xt_redist_collection_static.c

src/xt_redist_internal.h

src/xt_redist_single_array_base.c