Windows CE support : using standard _WIN32_WCE flag instead of WINCE

- when compiling for Windows CE, a C++ project must define the preprocessor definitions: UNDER_CE=$(CEVersion),_WIN32_WCE=$(CEVersion)
- choosing the "_WIN32_CE" form for uniformization with "_WIN32" and "_WIN32_WINNT" already used in libzmq (boost is using both forms)
- see http://msdn.microsoft.com/en-us/library/ee479161(v=winembedded.60).aspx

src/thread.cpp

@@ -43,7 +43,7 @@ void zmq::thread_t::start (thread_fn *tfn_, void *arg_)
 {
     tfn = tfn_;
     arg =arg_;
-#if defined WINCE
+#if defined _WIN32_WCE
     descriptor = (HANDLE) CreateThread (NULL, 0,
         &::thread_routine, this, 0 , NULL);
 #else