Yes, you simply need to connect the two computers with a cross-over Ethernet cable, and then setup the actual networking aspect. Actually, as far as I know all Macs since the G3's have had auto-sensing network cards, so it should
be able to work with either a cross-over or a patch cable, as the Mac's network card will adjust accordingly.
Give both machines static IPs (on the Slackware machine, only the Ethernet card connected to the Mac needs the static IP), make sure they can ping each other over the network, and then add add the following to your /etc/rc.d/rc.local file:
iptables --table nat --append POSTROUTING --out-interface ppp0 -j MASQUERADE
iptables --append FORWARD --in-interface eth0 -j ACCEPT
(You might need to adjust the interface names there)
Then make /etc/rc.d/rc.ip_forwarding executable with the command:
chmod +x /etc/rc.d/rc.ip_forward
On boot, your machine should setup the NAT between your dial up connection and Ethernet card, and then enable IP forwarding in the kernel. As long as you have the Mac setup with proper TCP/IP settings, you would be able to connect to the Internet through the Slackware machine.