[Bug libstdc++/69350] New: 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

            Bug ID: 69350
           Summary: Don't define the C99 <math.h> functions in -std=c++98
                    mode
           Product: gcc
           Version: 5.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

std::signbit etc. shouldn't be defined in C++98 mode.

They shouldn't be in namespace std, but for -std=gnu++98 (not -std=c++98) we
might want to leave them defined in the global namespace (that won't be
possible for GNU/Linux unless/until we stop defining _GNU_SOURCE implicitly).