Makefile to generate libJRiftLibrary.jnilib in Linux x86_64

I change the makefile in JRiftLibrary to compile the jnilib in Linux x86_64

I had to export OCULUS_SDK and JAVA_HOME:

In Fedora 18:
```
export JAVA_HOME=/usr/lib/jvm/java-1.7.0-openjdk.x86_64
export OCULUS_SDK=/home/<user>/OculusSDK
```