Microsoft implemented fopen_s some time ago. According to http://en.cppreference.com/w/c/io/fopen, it was made part of the C11 standard. (Forgive me if they're wrong.) GCC v5.1.0 doesn't seem to support it. grep fopen_s -r * through its source finds nothing. grep'ing through /usr/include (gcc 4.9.2) and /usr/local/include (gcc 5.1.0) only finds it in /usr/include/wine/msvcrt/{stdio.h & wchar.h}
These should be provided by your libc if at all. Also note the _s functions are an optional part of C11 and are not required.
(In reply to Andrew Pinski from comment #1) > These should be provided by your libc if at all. This means report it to glibc if you use GNU/Linux. Apple if you use Mac OS X, etc.