[phobos] std.concurrency unittests won't compile

[Lars Tandle Kyllingstad]

$ make unittest
...
Testing generated/posix/debug/unittest/std/concurrency
std/concurrency.d(636): Warning: statement is not reachable
std/concurrency.d(636): Warning: statement is not reachable
make[1]: *** [generated/posix/debug/unittest/std/concurrency] Error 1


The fix is easy, just turn the

        static if ...
        static if ...
        ...

starting at line 617 into

        static if ...
        else static if ...
        else ...

but I'm reluctant to do it myself, seeing how std.concurrency seems to
be under heavy development these days.

-Lars