cmake: only add -Wl,--undefined=WinMain when building an executable

cmake/test/main_lib.c

@@ -0,0 +1,33 @@
+#include "SDL.h"
+#include <stdio.h>
+
+#include EXPORT_HEADER
+
+#if defined(_WIN32)
+#include <windows.h>
+BOOL WINAPI DllMain(HINSTANCE hinstDLL, DWORD fdwReason, LPVOID lpvReserved) {
+    return TRUE;
+}
+#endif
+
+int MYLIBRARY_EXPORT mylibrary_init(void);
+void MYLIBRARY_EXPORT mylibrary_quit(void);
+int MYLIBRARY_EXPORT mylibrary_work(void);
+
+int mylibrary_init(void) {
+    SDL_SetMainReady();
+    if (SDL_Init(0) < 0) {
+        fprintf(stderr, "could not initialize sdl2: %s\n", SDL_GetError());
+        return 1;
+    }
+    return 0;
+}
+
+void mylibrary_quit(void) {
+    SDL_Quit();
+}
+
+int mylibrary_work(void) {
+    SDL_Delay(100);
+    return 0;
+}