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