proxygen: fix shared libs build

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

build/fbcode_builder/specs/proxygen.py

@@ -24,6 +24,8 @@ def fbcode_builder_spec(builder):
             # These 2 are set to ON in `proxygen_quic.py`
             "BUILD_QUIC": "OFF",
             "BUILD_TESTS": "OFF",
+            # For bistro
+            "BUILD_SHARED_LIBS": "OFF",
         },
     )