Add JNI headers from GCC 4.9.2

Cross builds can't use JNI headers from the build machine, since Linux
installs of the JDK don't include the Windows version of jni_md.h.
We've been working around this by using GCJ's portable implementation of
the JNI headers in the GCC include directory, but GCJ is starting to be
removed from Linux distributions (e.g. Fedora 21).  Add a copy of the
Classpath JNI headers for use in cross builds.
Use bundled JNI headers for cross builds