b2sum: port to HP-UX aCC

b2sum: port to HP-UX aCC

Its support for the -include option is flaky.  Problem reported by
Michael Osipov (Bug#35650).  Plus, we could run into other
compilers that don’t support any option like -include.  Change the
code so that -include is not needed.  Although this causes us to
depart from the upstream version, we’re already doing that for
other reasons.
* configure.ac (USE_XLC_INCLUDE): Remove, as there’s no
guarantee a compiler will support something like -include.
* src/blake2/b2sum.c [HAVE_CONFIG_H]: Include <config.h>.
* src/local.mk (src_b2sum_CPPFLAGS): Add -DHAVE_CONFIG_H.
Do not use -include or a substitute.