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.
