Make --with-threads=none mean, that we want thread support, but no default

Make --with-threads=none mean, that we want thread support, but no default

2001-10-31  Sebastian Wilhelmi  <wilhelmi@ira.uka.de>

	* configure.in: Make --with-threads=none mean, that we want thread
	support, but no default thread implementation. This also was the
	original intention, but disapeared around 1998... So it doesn't
	seem to be the most requested feature, but we went far to make it
	possible in the code, so I resurrected this feature.

	* INSTALL.in: Describe the --enable-gc-friendly, --disable-threads
	and --with-threads options of configure.