Use Catboost cflags from Arcadia rather then exprot toool hardcode

ref:0d2672cbde86f673a498641c83483cd408135c40

build/conf/opensource.conf

@@ -2,6 +2,8 @@ EXPORT_CMAKE=no
 
 when ($CATBOOST_OPENSOURCE == "yes") {
     OPENSOURCE=yes
+    CFLAGS+=-DCATBOOST_OPENSOURCE=yes
+    CXXFLAGS+=-DCATBOOST_OPENSOURCE=yes
 }
 
 when ($OPENSOURCE) {