Well, it seems that the updated package for gnutls does not include the libgnutls-config utility, while the package for Slack13 does, 32 or 64 bit doesn't matter.
The difference between original and updated packages is quite obvious:
alvaro@alvaro:~# tar tvf gnutls-2.6.6-x86_64-2.tar | grep bin
drwxr-xr-x root/root 0 2009-05-23 01:37 usr/bin/
-rwxr-xr-x root/root 66264 2009-05-23 01:37 usr/bin/gnutls-serv
-rwxr-xr-x root/root 66024 2009-05-23 01:37 usr/bin/gnutls-cli
-rwxr-xr-x root/root 1704 2009-05-23 01:37 usr/bin/libgnutls-extra-config
-rwxr-xr-x root/root 1673 2009-05-23 01:37 usr/bin/libgnutls-config
-rwxr-xr-x root/root 18176 2009-05-23 01:37 usr/bin/psktool
-rwxr-xr-x root/root 23696 2009-05-23 01:37 usr/bin/srptool
-rwxr-xr-x root/root 109288 2009-05-23 01:37 usr/bin/certtool
-rwxr-xr-x root/root 54848 2009-05-23 01:37 usr/bin/gnutls-cli-debug
-rw-r--r-- root/root 758 2009-05-23 01:37 usr/man/man3/gnutls_hex2bin.3.gz
alvaro@alvaro:~# grep bin /var/log/packages/gnutls-2.8.4-x86_64-1_slack13.0
usr/bin/
usr/bin/gnutls-cli-debug
usr/bin/certtool
usr/bin/gnutls-serv
usr/bin/gnutls-cli
usr/bin/psktool
usr/bin/srptool
usr/man/man3/gnutls_hex2bin.3.gz
So my quick 'n' dirty solution was to get back to stock Slack13 package, compile FileZilla, and only then upgrade to gnutls-2.8.4
I don't know if it is something wrong with the upgraded package, a change in gnutls source code, or what. But this way I got it working. I'd like to know which programs depend on gnutls and if any of those that I may want to compile will complain about the same thing...
Thanks for your reply, it was enough to force me to do some more research