You'll need a suitable Palm developement environment. Weasel was created with Palm SDK 5.2, so you'll need a copy of that too. I used Debian/GNU Linux for development with PRC-Tools 2.1.
Weasel uses the PalmZLib shared library to do decompression on the Palm. You don't actually need this to compile Weasel, but you will, of course, need it to run the program. You can get PalmZLib at http://palmzlib.sourceforge.net.
makeztxt uses zLib v1.1.4 (homepage). You will need to have zLib compiled for your HOST machine. All Linux distributions as well as most other Unices come with zLib, though it is possible you may be lacking the zLib header files.
You should look in the Makefile to make sure the program names and paths are okay.
You should now have weasel-[language].prc and makeztxt. If you're messing with the source, then maybe you want to help. If you have any problems, feel free to email me at email@example.com.