[Bug libstdc++/69350] Don't define the C99 functions in -std=c++98 mode

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69350

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
The simplest solution might be something like:

--- a/libstdc++-v3/include/c_global/cmath
+++ b/libstdc++-v3/include/c_global/cmath
@@ -840,7 +840,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       return __builtin_isunordered(__type(__x), __type(__y));
     }

-#else
+#elif !defined(__STRICT_ANSI__)

   template<typename _Tp>
     inline typename __gnu_cxx::__enable_if<__is_arithmetic<_Tp>::__value,