[Bug libstdc++/82346] [5.5 Regression] String is not detected as a part of std

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82346

--- Comment #7 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The condition for std::to_string being declared in gcc-5 is:

#if __cplusplus >= 201103L && defined(_GLIBCXX_USE_C99)

So presumably _GLIBCXX_USE_C99 is false. If you're using glibc 2.26 you might
have hit https://sourceware.org/bugzilla/show_bug.cgi?id=22146 and so need a
glibc fix so that libstdc++ correctly detects C99 support.