If Evolution requires Gnome 1.4, you should try to install the correct libraries from Slackware 8.1. I have had the same problem while trying to compile other applications and solved it by installing the following:
from /slackware-8.1/slackware/l/
gtk+-1.2.10-i386-2.tgz
lib*.tgz
from /slackware-8.1/slackware/gnome/
gnome-libs-1.4.1.7-i386-1.tgz
gtkhtml-1.0.2-i386-1.tgz
There may be more, but you get my drift: each time the compilation complains about a missing lib, just look for it within the Slackware 8.1 tree structure, and install it as root.
Please note that this will not create any problem for your Slackware 9.0 machine, since different versions of the same library can co-exist peacefully on your machine...
Hope this helps!