[Bug libstdc++/109818] std::trunc() requires a <cmath> hack after building DJGPP

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109818

--- Comment #29 from Jonathan Wakely <redi at gcc dot gnu.org> ---
(In reply to Janez Zemva from comment #27)
> forcing glibcxx_cv_c99_math_tr1=yes solved this issue for me. The C99
> compliance test is really strict, even openlibm fails the test (it fails to
> define float_t and double_t, but this can be hacked around).

It's precisely as strict as it needs to be: it checks that every name that will
be subject to a using-declaration is present. Because otherwise compiling
<cmath> will give errors.

But supporting openlibm would be nice, so I'll separate the checks for those
typedefs.