[patch] fix thread test cases for solaris.

[Paolo Carlini]

Paolo Carlini wrote:
> Let's have these three options for libstdcxx-librt: no, auto, yes. In
> the first case no additional libraries, ever; in the second case only
> posix4 (it's safe), if it helps; in the third case rt too, if it helps.
> The default is auto. Seems consistent to me.
>   
Oops, forgot the link tests issue.

Ok, let's call the configure time option something like
libstdcxx-time=(no, yes, rt), and the behavior exactly the same as now.

Paolo.