Allow forcing value for FOLLY_FUTURE_USING_FIBER in folly-config.h

Summary:
Let clients choose whether to use fibers or not, but default
to the same autodetected setting as prior logic.

Reviewed By: yfeldblum

Differential Revision: D5585605

fbshipit-source-id: 9a4bf9f9bc23b4e15601f30c40602f21798c9685

folly/futures/Future-inl.h

@@ -28,12 +28,14 @@
 #include <folly/futures/Timekeeper.h>
 #include <folly/futures/detail/Core.h>
 
+#ifndef FOLLY_FUTURE_USING_FIBER
 #if FOLLY_MOBILE || defined(__APPLE__)
 #define FOLLY_FUTURE_USING_FIBER 0
 #else
 #define FOLLY_FUTURE_USING_FIBER 1
 #include <folly/fibers/Baton.h>
 #endif
+#endif
 
 namespace folly {