[c++] Keep tm, div_t, ldiv_t, lconv mangling on Solaris (PR libstdc++-v3/1773)

[Rainer Orth]

Hi Paolo,

> All in all it seems to me that we are pretty close to be able to fix this
> old issue now and we are even in Stage 1, thus we can afford to take a bit
> of risk and handle possible fallout, thus I would recommend we do for now
> the above preprocessor dance (we are talking only about 4 instances) but
> controlled by a macro in os_defines.h, as Rainer correctly did already
> elsewhere. Rainer, can you test such change?

I could, but am a bit reluctant to do so since such a fix feels quite
fragile, and `fixes' the Solaris headers in many places where they are
completely correct.  I'll also have to touch <time.h>, <wchar.h>,
<stdlib.h>, and <locale.h>, that all have using std::tm etc. clauses.

There might be an alternative implementation that is less invasive to
the C++ frontend, though: add

	&& TARGET_DECL_NAMESPACE_STD_P (decl)

in write_unscoped_name, defaulting to true, override it in sol2.h (which
gets included via tm.h) and have the remaining logic in a new sol2-cxx.c
file.  This way, the impact on the C++ frontend is minimal and we don't
have to resort to fragile fixincludes hacks.

Does this sound like a acceptable/viable alternative?

Thanks.
        Rainer

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Rainer Orth, Center for Biotechnology, Bielefeld University