fix: weejobs macos build issues

[remoe]

• std::mutex doesn't have a string as a constructor.
• initialize of a reference doesn't compile on clang

[gwaldron]

Hi Remo. Both of these occurred in clang because clang is evaluating templates before they are used.

Anyway, the first is a bug and I have fixed it in a separate push. Thanks.

The second however is not a problem with a default reference value, that is legal C++. The problem is that the struct context type is being forward-declared (since it's evaluating the template before it's used) and you cannot assign a default value to an argument of a forward-declared type. So I will try to rearrange the code to prevent this.

[remoe]

This is now solved by:

43237fd
e00f167