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

[redi at gcc dot gnu.org]

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

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
I'm inclined to close this one as WONTFIX.

We should fix PR 11196 but I don't really care about this C++98 non-conformance
now, and we've been doing it this way since before C++11 was even a thing.
Defining those <math.h> functions before C++11 is not the same as dumping a
load of totally non-standard GNU extensions into the global namespace.