don't warn that WIN32 is not defined on cygwin

CMakeLists.txt

@@ -5,6 +5,9 @@
 
 cmake_minimum_required(VERSION 2.8 FATAL_ERROR)
 
+# do not warn about non-definition of WIN32 on cygwin
+set(CMAKE_LEGACY_CYGWIN_WIN32 0) 
+
 # if CMAKE_BUILD_TYPE is specified use it; otherwise set the default
 # build type to "RelWithDebInfo" ("-O2 -g" with gcc) prior to calling
 # project()