Rewriting the build system would be a waste of time. We have the tools to use Abiword's current one, which will be much less work.
Here is how I would do it (but it's just one way...)
1) Get latest AbiWord sources
2) Try to get it to compile
3) Find out that there are many dependencies (at least, I know they use various libraries for loading and saving files in different formats)
4) Port the libraries
5) Get Abiword to compile. Not necessarily remove the non-Haiku code, but disable it and add "stubs" (empty functions) for the Haiku implementation. For later help, the stubs can do something like print a message to the output ("I'm $functionName and I'm not implemented!")
6) You will get a binary that runs, but does nothing or crashes.
7) See which stubs are called, and start filling them in one after another, until you get the whole program running.
8) Later on we can see about more integration with Haiku: using translators, etc, and maybe adding new features.
The existing port of Abiword is an old version and there are some broken design decisions in it. But it could still serve as a reference for some things. In particular, it shows how much code is needed.
For reference, the code for that is here: https://github.com/HaikuArchives/AbiWord
The history of this repo starts with an import of AbiWord sources, hopefully unmodified. Then, there are several changes to get it to run on BeOS, then on Haiku.