Skip library postfix on OSX.
Build improvements and Javadoc generation:

- Parameterized more paths in the build script.
- Enabled bin & generated paths to be set outside the repo (can be absolute paths). This allows to have the source on an SSD and the (non-SSD friendly) artifacts on a conventional HDD.
- Added demo target. Usage example: ant demo -Dclass=org.lwjgl.demo.glfw.Events -Djvmargs=-Dorg.lwjgl.util.Debug=true
- Added Javadoc generation target (ant javadoc). Also improved the javadoc template DSL.
Javadoc improvements using the new APIs.