FPC certainly qualifies as a cross platform OS, and I believe it runs well on MacOS?? I have never tried it. In the past it was always a big deal to convert the source, but nowadays, with the ISO 7185 mode, that should be a lot less work. It really depends on FPC. I know with some other compilers, the ISO 7185 switch enabled standard mode, but locked you into "ISO 7185 jail", ie, made it inconvienent or impossible to use their extensions unless you drop the option, in which case most of your source would not compile, etc. Perhaps someone with FPC experience could weigh in here.
The problem are keywords. E.g. uses is valid identifier in ISO Pascal so it cannot be used as keyword to include units etc. Besides this, FPC does not disable non ISO extensions in ISO mode.