So I took at look into Build Rules in XCode. My main trouble within using them was finding the environment variables, even googling them doesn’t turn up much information.
I think the output files act as an error check, if the output file fails to appear then the tool failed.
One other thing I had an issue with was the source files list. I tried comma and semi colon delimiters, and spaces, but the TGA file kept giving the warning on Tree1.tga. I have yet t0 find the a way to do this without multiple entries. And I’m thinking a separate tool for building files may be easier.