Users of your library *should* call "-lpthread".
If you want, you could take all the stuff lib your version of "libpthread.a" (ar xv) and add it all to your library. But that would be unwise - it would be a maintenance headache for you, and for your users.
Alternatively, you could simply ship a copy of "libpthread" (atatic or shared) with your library (as a courtesy for those users who might not happen to have their own local copy).
IMHO .. PSM