cmake: use WINDOWS instead of WIN32.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 5b0f7f8..608e9e2 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -212,7 +212,7 @@ set(SDL_CFLAGS "")
# When building shared lib for Windows with MinGW,
# avoid the DLL having a "lib" prefix
-if(WIN32)
+if(WINDOWS)
set(CMAKE_SHARED_LIBRARY_PREFIX "")
endif()