Fix issue when building with Ant and system default encoding is not UTF-8

In my case, fixes the following issue:
    [javac] C:\test\jeromq-master\src\main\java\org\zeromq\ZMQ.java:1541: error: unmappable character for encoding Cp1251
    [javac]      * Starts the built-in ├?MQ proxy in the current application thread.
    [javac]
    [javac] 1 error