backingstore: conditionalize use of "fb" feature

Summary: Don't set the "fb" cargo feature via cmake unless we are in fb build.

Reviewed By: quark-zju

Differential Revision: D33781554

fbshipit-source-id: 39c61dc1631ebfd8a2c858b5e214c6a09ce0b005

build/fbcode_builder/manifests/eden

@@ -70,8 +70,7 @@ ENABLE_GIT=OFF
 
 [cmake.defines.fb=on]
 USE_CARGO_VENDOR=ON
-INCLUDE_FB_INTEGRATION_LIB=ON
-INCLUDE_FB_DOCTOR_LIB=ON
+IS_FB_BUILD=ON
 
 [depends.environment]
 EDEN_VERSION_OVERRIDE