fossil is licensed under the GNU GPL v2.
unfortunately, openssl, while free, has a GPL-incompatible license.
this means that distributors (like, say, debian) cannot ship fossil binaries linked to openssl.

UNLESS you add an "exception" clause to the fossil license.
Many programs have such an exception clause, e.g., wget has one,
which I include below.  (This is an FSF-approved thing to do, BTW;
the wording below is the wording the FSF uses when they add such
a clause, and which they have recommended to others.)

"In addition, as a special exception, the Free Software Foundation
gives permission to link the code of its release of Wget with the
OpenSSL project's "OpenSSL" library (or with modified versions of it
that use the same license as the "OpenSSL" library), and distribute
the linked executables.  You must obey the GNU General Public License
in all respects for all of the code used other than "OpenSSL".  If you
modify this file, you may extend this exception to your version of the
file, but you are not obligated to do so.  If you do not wish to do
so, delete this exception statement from your version."

If the above is unclear (or unconvincing!) please do not hesitate to email.
--
Barak A. Pearlmutter