This makes the main source code look a little cleaner, at the expense of
background magic happening in the configure-generated config.h. But this
magic was already happening for _GNU_SOURCE, __EXTENSIONS__ and the like.
The only wrinkle is that newer versions of NcursesW cause ./configure to
include "-D_XOPEN_SOURCE=600" as part of the compilation command line--on
systems that support _XOPEN_SOURCE=700. The AC_DEFINE/AC_DEFINE_UNQUOTED
macros automatically comment out any "#undef" lines in config.h, so the
file lib/xopen_source.h works around this limitation. It is
automatically included by config.h.
Previous versions of the GNU Portability Library, and versions of the GNU
C Library as released with various Linux distributions, used strings no
longer present in the current version of the GNU Portability Library.
When used on systems with those strings, Star Traders could not provide
appropriate translations. This patch fixes that problem.
