Skalarkify flags macros.

Reviewed By: adamjernst

Differential Revision: D6528242

fbshipit-source-id: f809cfb853d6615a1aa47e145063ef1d7f5838be

Author: ttsugriy

ReactCommon/cxxreact/BUCK

@@ -1,3 +1,5 @@
+include_defs("xplat//configurations/buck/apple/flag_defs.bzl")
+
 include_defs("//ReactCommon/DEFS")
 
 CXX_LIBRARY_COMPILER_FLAGS = [
@@ -7,7 +9,7 @@ CXX_LIBRARY_COMPILER_FLAGS = [
 
 if THIS_IS_FBOBJC:
   inherited_buck_flags = STATIC_LIBRARY_IOS_FLAGS
-  CXX_LIBRARY_COMPILER_FLAGS += inherited_buck_flags.get_flag_value('compiler_flags')
+  CXX_LIBRARY_COMPILER_FLAGS += flags.get_flag_value(inherited_buck_flags, 'compiler_flags')
 
 rn_xplat_cxx_library(
     name = "module",