pthread stubs not provided by native libc

 This library provides weak aliases for pthread functions not provided
 in libc or otherwise available by default. Libraries like libxcb rely
 on pthread stubs to use pthreads optionally, becoming thread-safe when
 linked to libpthread, while avoiding any performance hit when running
 single-threaded. libpthread-stubs supports this behavior even on
 platforms which do not supply all the necessary pthread stubs. On
 platforms which already supply all the necessary pthread stubs, this
 package has no content.