Don't link with libgnutls-openssl, which is now GPLv3+.

Don't link with libgnutls-openssl, which is now GPLv3+.

ELinks used to call the MD5 code in libgnutls-openssl, part of
GNUTLS-EXTRA, which was licensed under GNU GPL version 2 or later.
In GnuTLS 2.2.0 however, the license of GNUTLS-EXTRA has been changed
to GNU GPL version 3 or later.  This is no longer compatible with
GNU GPL version 2 as used in the current ELinks, because GPLv2 clause
2. b) requires the whole work to be licensed under GPLv2, and GPLv3
does not allow that.

If anyone is still using a pre-2.2 GnuTLS, he or she can tweak
configure.in to check the version or just assume it's old enough.
There is not much reason to do so though, as including the MD5 code
in ELinks seems to cost only about 4 kilobytes on i686.
(cherry picked from commit 9ca0182ec6ffa5f652a3d45529e82ee31329f301)