Summary:
Right now proxygen hardcodes a static build when using cmake  and
ignores BUILD_SHARED_LIBS. Fix that, and enable PIE on the shared libs so they
can be linked properly

Closes: https://github.com/facebook/proxygen/issues/335

Reviewed By: mjoras, lnicco

Differential Revision: D24787944

fbshipit-source-id: 7a654af7cb43227ca913a1bed67f2432703a343d