diff --git a/reactos/cmake/gcc.cmake b/reactos/cmake/gcc.cmake
index ad2e5ad88bb..8d6460477a9 100644
--- a/reactos/cmake/gcc.cmake
+++ b/reactos/cmake/gcc.cmake
@@ -20,8 +20,7 @@ add_compile_flags("-pipe -fms-extensions")
 #file(TO_NATIVE_PATH ${REACTOS_SOURCE_DIR} REACTOS_SOURCE_DIR_NATIVE)
 #workaround
 set(REACTOS_SOURCE_DIR_NATIVE ${REACTOS_SOURCE_DIR})
-set(TEST_ENV $ENV{windir})
-if(DEFINED TEST_ENV)
+ if(CMAKE_HOST_SYSTEM_NAME MATCHES Windows)
 string(REPLACE "/" "\\" REACTOS_SOURCE_DIR_NATIVE ${REACTOS_SOURCE_DIR})
 endif()
 add_compile_flags("-fdebug-prefix-map=${REACTOS_SOURCE_DIR_NATIVE}=ReactOS")