In the 1990's, dialects of C that had // comments were C++, various dialects accepted by C compilers having // comments as a nonconforming extension over C90, and C compilers supporting draft ISO C features.
Changing dialects just for the sake of comments is silly; the right choice will usually be "don't use those comments".
In C programming, I still don't use // comments today, to keep my code strictly compatible with C90 (in my opinion, the best C dialect to date). I have been a C++ developer for many years and of course used // comments; I have nothing against them, and in fact wouldn't design a language syntax that doesn't have a "from here to the end of the line" comment feature.
(Text filtering to obtain C features is not unheard of, by the way. The CLISP sources use something called "varbrace": a text processor which provides mixed declarations and statements.)