jni.h compatibility

[Tom Tromey]

I've found that some JNI code uses JNIEXPORT and JNICALL macros.  I
think we need to add these to our jni.h for compatibility.

For Unix platforms this isn't a problem; they can just be defined as
empty.  For Windows, though, I think they might need a real
definition.  Adam, do you know how they should be defined?

They are used like this:

    JNIEXPORT jint JNICALL foo (...);

Tom