Linking with -pthread only if linking with a .so that needs it

[Florian Weimer]

* Sam Varshavchik:

> libcxx.so gets built by gcc.7.3.1 with -std=c++17 -O2
> -fvisibility-inlines- hidden, -fno-omit-frame-pointer, and
> -pthread. It contains some static variables that are instantiated
> templates. It contains other static variables whose constructors
> invoke the instantiated templates' methods that construct and
> destruct std::unique_lock<std::mutex> of std::mutexes that are
> static variables of the instantiated templates. Static
> initialization of libcxx.so does not create any execution thread.

What's the binding type for __pthread_key_create in libcxx.so?  I
assume that's the symbol used as a key for __gthread_active_p.