Add --without-libproxy and --without-stoken options to configure

"Automagic dependencies" can cause trouble for source-based distributions
like Gentoo [1], so we will provide a way to disable them.

As written, this does not honor --with-libproxy or --with-stoken.

[1] http://www.gentoo.org/proj/en/qa/automagic.xml

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>