[gnufx]

Rather beside the point, but for people who aren't used to this, the usual dodges are first to search for the "final text" produced by the relevant standard committee (I have C and Fortran stashed, but I once got work to pay for the real C one). If that fails maybe consult the manual of a well-documented, high-quality implementation; that should tell you what versions(s) of the relevant standard(s) it supports and clearly mark extensions of the standard.