diff --git a/xs/CMakeLists.txt b/xs/CMakeLists.txt
index 68f061f25..4d6712027 100644
--- a/xs/CMakeLists.txt
+++ b/xs/CMakeLists.txt
@@ -5,9 +5,9 @@ set(CMAKE_CXX_STANDARD_REQUIRED ON)
 # Add our own cmake module path.
 list(APPEND CMAKE_MODULE_PATH ${PROJECT_SOURCE_DIR}/cmake/modules/)
 
-if (UNIX)
+if (CMAKE_SYSTEM_NAME STREQUAL "Linux")
     # Workaround for an old CMake, which does not understand CMAKE_CXX_STANDARD.
-    set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 -Wall")
+    add_compile_options(-std=c++11 -Wall)
 endif()
 
 # Where all the bundled libraries reside?