[v3] Handle different versions of Solaris 8 <iso/math_iso.h>, <iso/stdlib_iso.h>

[Paolo Carlini]

Hi,
> Ok for mainline if bootstraps pass?
Not a comment strictly about this patch, but why we have things like #if 
__cplusplus >= 199711L anywhere? For sure the library is not supposed to 
be used together with old C++ front-ends.

Paolo.