autoconf: Add support for setting CURLDIR, OPENSSLDIR, EXPATDIR

autoconf: Add support for setting CURLDIR, OPENSSLDIR, EXPATDIR

Add support for --with-openssl=PATH and --without-openssl,
--with-curl=PATH and --without-curl, --with-expat=PATH and
--without-expat ./configure options, each setting or unsetting
appropriate NO_PACKAGE and if called with argument also PACKAGEDIR
(of which only CURLDIR is documented in Makefile)

All above options are supported as override to autodetection; more to
come in the same style (override to autodetection), so moved the bulk
of comments for site configuration down.

Needs review by someone well versed in autoconf and m4.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>