cmake: set windows version to vista on mingw-w64

mingw-w64 defaults to xp, so when building we set it to vista because
function CancelIoEx is used in ipc-util/pipe-windows.c.

http://msdn.microsoft.com/en-us/library/windows/desktop/aa363792.aspx

CMakeLists.txt

@@ -57,6 +57,10 @@ if(MSVC)
 		set(CMAKE_MODULE_LINKER_FLAGS "${CMAKE_MODULE_LINKER_FLAGS} /SAFESEH:NO")
 	endif()
 else()
+	if(MINGW)
+		set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -D_WIN32_WINNT=0x0600 -DWINVER=0x0600")
+		set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -D_WIN32_WINNT=0x0600 -DWINVER=0x0600")
+	endif()
 	set(CMAKE_C_FLAGS_DEBUG "-DDEBUG=1 -D_DEBUG=1 ${CMAKE_C_FLAGS_DEBUG}")
 	set(CMAKE_CXX_FLAGS_DEBUG "-DDEBUG=1 -D_DEBUG=1 ${CMAKE_C_FLAGS_DEBUG}")
 endif()