Stop explicitly disabling GNU compiler extensions.

Summary:
GNU compiler extensions (specifically `gnu++11`) are required for our use of `##__VA_ARGS__` in serialization macros, else compilation fails. Temporarily disabling until we find a workaround.

(Note: this ignores all push blocking failures!)

Reviewed By: vineelpratap

Differential Revision: D13544391

fbshipit-source-id: e9c008476cd45dc0afcf9a5475eef6569abed513

readthe docs integration 2