diff --git a/src/os/os2/CMakeLists.txt b/src/os/os2/CMakeLists.txt
--- a/src/os/os2/CMakeLists.txt
+++ b/src/os/os2/CMakeLists.txt
@@ -1,5 +1,8 @@
 add_files(
     os2.cpp
-    os2_main.cpp
     CONDITION OPTION_OS2
 )
+
+if(OPTION_OS2)
+    target_sources(openttd PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/os2_main.cpp)
+endif()