Add a signal handler for SIGINT and SIGTERM, as well as sprinklings of
checks to abort_game. Although Ncurses DOES define handlers for SIGINT
and SIGTERM, they do not always seem to work correctly under many
operating systems.
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/.
