[Bug c++/70217] Cygwin 2.4.1 x86_64/GCC 5.3.0 can no longer compile a program with std=c++03

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70217
--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
We can probably do something in os_defines.h like:

#if __cplusplus < 201103L && __STRICT_ANSI__
#undef _GLIBCXX_USE_C99_STDIO
// etc.
#endif