6.28. C++ Style Comments
In GNU C, you may use C++ style comments, which start with // and
continue until the end of the line. Many other C implementations allow
such comments, and they are included in the 1999 C standard. However,
C++ style comments are not recognized if you specify an -std
option specifying a version of ISO C before C99, or -ansi
(equivalent to -std=c89).