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.
In particular, the gnulib-tool utility now maintains lib/Makefile.am, as
well as many files in the m4 and lib directories. The file
m4/gnulib-cache.m4 allows the Gnulib files to be refreshed. See
http://www.gnu.org/software/gnulib/.
Rewrite the input to autoconf: add the definition of PACKAGE_AUTHOR,
add checks for Standard C and for C compiler warnings (via the file
m4/ax_cflags_warn_all.m4 from the GNU Autoconf Archive), rearrange
various macros and the order of Makefile creation.
