Skip to content drop dependency `pthread-stubs` on Solaris

Alan Coopersmith requested to merge alanc/libxcb:pthreads into master

On Solaris 10 and later, the pthread functions are directly in libc, and libpthread only has metadata to redirect calls from it to the libc functions.

On Solaris 9 and older (no longer supported), libc contained its own thread stubs that libpthread then overrode.

Merge request reports