Enable gnulib-tool's --with-tests option.

Enable gnulib-tool's --with-tests option.

* bootstrap: After all is done, transform the generated gnulib-tests/gnulib.mk.
* bootstrap.conf (gnulib_tool_option_extras): Set it here.
* gnulib-tests/Makefile.am: New file.
* Makefile.am (SUBDIRS): Add gnulib-tests.
* configure.ac (AC_CONFIG_FILES): Add gnulib-tests/Makefile.
* m4/jm-macros.m4 (coreutils_MACROS) [ARGMATCH_DIE_DECL]: Omit the
"extern " prefix to "void usage ()", so that the macro string value can
be used to construct a function definition in gnulib's test-argmatch.c.