Fix downversioning of _WIN32_WINNT

Forcing the _WIN32_WINNT when the environment is already forcing a value
results in a lot of warning.

If the user compiles for a higher version of Windows, we don't need to force
compilation for Vista.

CMakeLists.txt

@@ -291,7 +291,11 @@ if(WIN32)
 	if(ENABLE_INET_PTON)
 		set(CMAKE_REQUIRED_LIBRARIES ws2_32)
 		check_function_exists(inet_pton HAVE_INET_PTON)
-		add_definitions(-D_WIN32_WINNT=0x0600)
+		check_c_source_compiles("#include <windows.h>\n#if !defined(_WIN32_WINNT) || _WIN32_WINNT < 0x0600\n#error NOPE\n#endif\nint main(void) {return 0;}" AT_LEAST_VISTA)
+		if(NOT AT_LEAST_VISTA)
+			# force targeting Vista
+			add_definitions(-D_WIN32_WINNT=0x0600)
+		endif()
 	else()
 		add_definitions(-D_WIN32_WINNT=0x0501)
 	endif()