Actually, this is needed for building with Visual Studio with both /MT and /MD.

With the previous change, I get:
1>     Creating library C:\projects\SDL\VisualC\Win32\Debug\SDL2.lib and object C:\projects\SDL\VisualC\Win32\Debug\SDL2.exp
1>LINK : error LNK2001: unresolved external symbol __DllMainCRTStartup@12

src/SDL.c

@@ -451,7 +451,9 @@ SDL_GetPlatform()
 #endif
 }
 
-#if defined(__WIN32__) && defined(__WATCOMC__) && defined(BUILD_DLL)
+#if defined(__WIN32__)
+
+#if !defined(HAVE_LIBC) || (defined(__WATCOMC__) && defined(BUILD_DLL))
 /* Need to include DllMain() on Watcom C for some reason.. */
 
 BOOL APIENTRY
@@ -469,4 +471,6 @@ _DllMainCRTStartup(HANDLE hModule,
 }
 #endif /* building DLL with Watcom C */
 
+#endif /* __WIN32__ */
+
 /* vi: set sts=4 ts=4 sw=4 expandtab: */