nobs should stop appending -lpthread

Currently nobs unconditionally blindly appends -lpthread to links for every unit test it builds, and to the LIBFLAGS in upcxx-meta, thus to all application builds.

This is bad for several reasons:

It's not portable. On some platforms the proper spelling for this option differs (eg on BSD the correct usage is -pthread with no -l)
-lpthread is not sufficient to get thread-safe system libraries. Many system headers also require defines like -D_THREAD_SAFE or -D_REENTRANT to provide correct operation in a thread-safe environment.
Nobs is appending it even for the single-threaded backend. The main reason a single-threaded backend exists in GASNet (and UPC++) is to excise this very dependency. Appending -lpthread means if some part of the runtime or app code accidentally adds a dependency on the pthread library (eg an unconditional use of a mutex), it's likely to just silently work and silently degrade performance.

GASNet's makefile fragments already add the appropriate flags for pthreads support in the GASNET_DEFINES and GASNET_LIBS for the PAR backend. This should already be correctly present in the parallel backend. nobs should just rely on that and stop ever adding -lpthread.

If there is a GASNet-free unit test that requires threading (uts_threads) then -lpthread should be added to just that test, with the understanding that test might not build portably.

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