Bug 1021: Add OculusSDK binding / Basic OVR support (Refine build exclusion)

common:
  - property 'oculusvr.sdk.available' is only set if sub-module 'oculusvr-sdk' is available
  - if 'oculusvr.sdk.available' is not set build.xml will not issue build-oculusvr.xml's 'all' target

build-oculusvr.xml:
  - propery 'oculusvr.build.enabled' is only enabled if 'oculusvr.sdk.available' is set
    and targer 'all' will only pass if set.
Bug 1021: Add OculusSDK binding / Basic OVR support (HowtoBuild.xml: Mention feature loss if not fetching submodules)