To see how cross platform C looked like in those early days, nothing like reaching out to books like "A book on C" from 1984 (Robert Edward Berry and B. A. E. Meekings), which has an implementation of RatC [0].

https://www.amazon.de/-/en/Robert-Berry/dp/0333368215/ref=sr...

Then there were Small-C and BDS C, as yet two other major subsets in those early days.

[0] - Similar in ideas to Ratfor but applied to a K&R C subset