Explicitly link to pthread

[marler8997]

Currently, phobos is using -defaultlib= to build itself, but is implicitly relying on its library dependencies to be added by dmd.

This change makes the phobos library dependencies explicit rather than relying on its dependencies to be hardcoded into the compiler, which in turn forces all D programs to link to those libraries as well.

This change is required to allow dlang/dmd#9831 to be merged, which removes these implicit libraries from being linked to ALL D programs that use DMD to link.

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Testing this PR locally

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[thewilsonator]

Ping me when this is good.

[marler8997]

I've verified that adding -L-lpthread fixes the step that the autotester was on when it failed with the DMD PR, however, there may be more libraries that need to be added but I don't know of an easy way to test that without using the CI builders...and I need both this PR and the DMD PR to be tested at the same time.

I know for sure we need -lpthread. We could integrate this one first and see if that's all we need, and make more PRs if we find any other libraries that need to be added.

The other libraries that may need to be added are -lm and -lrt. However, they could already be added by some other means. -ldl is another dependency but it is already being added explicitly in the makefile.

[marler8997]

Closing and re-opening to trigger autotester retry. It looked like it failed on windows because of a race condition between running executables and then removing them afterwards.

[marler8997]

This one should be ready, if we end up needing to add more libraries then I'll make another PR.

[wilzbach]

Makefile use tabs 🤷‍♂️

[marler8997]

good catch

[marler8997]

Been a few days, any objections to merge this now? Should we apply that label "merge in X days if no objections?" Do we still have that label?