Fix conflict on UNUSED() macro from OpenJDK 7

The OpenJDK 7 recently introduced the UNUSED() macro in their jni_md.h
header file, which is included from <jni.h>.  This causes a
compile-error if "constants.h", which defines our own UNUSED() macro, is
included as well as <jni.h>.

The OpenJDK UNUSED() macro is unsuitable for our own use, because it
prefixes the unused identifier with "UNUSED_" whereas we depend on the
parameter name remaining unchanged.

I have reported this as a Request for Enhancement with Oracle Java,
asking them to remove the UNUSED() macro, since it is not used by any
JNI or Java extension header files.  Review ID: JI-9013689.

In the meantime, constants.h now undefines UNUSED before defining it, so
including <jni.h> before "constants.h" will avoid a compile error.