[Bug libstdc++/61580] stoi function unknown on W7/Cygwin/x86_64

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61580

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The problem in this case is that newlib only defines C99 functions for C99 or
C++11, but the _GLIBCXX_USE_C99 tests in acinclude.m4 are compiled with
-std=gnu++98.

As discussed previously (on the mailing list IIRC) we need to test whether C99
functions are available in C++98 mode and test again whether they are available
in C++11 mode. For std::stoi() we don't care if they are missing in C++98 mode,
as long as they are present in C++11 mode (which is true for newlib).