Enable per-thread errno on Solaris

This addresses bug 4354 for - Phobos should expose per-thread errno - on Solaris systems.

On Solaris, without the MT errno definition, errno is always the errno value of the first thread of the processes.  
While porting druntime and Phobos, this was the source of a lot of frustration as unit tests were failing with impossible and non-sensical error messages due to showing the wrong error value.

Just use `-pthread` for all platforms instead please.