Patch to build static library (libgnuradio-osmosdr.a)

I have attached a patch for gr-osmosdr to build it as a static library (libgnuradio-osmosdr.a) as well as the normal shared lib. This is the same method we use in GNU Radio. The main difference here, is that the patch means that the static lib will always be built. In GNU Radio, we added a -DENABLE_STATIC_LIBS option to cmake to turn this on/off. Let me know if that's preferably and if you have specific styles for adding these options (if you can just point me to the right line if you have other flags of this nature). The static libs are used in Android and other embedded systems. It's not entirely necessary, but it sure makes some things a heck of a lot simpler. Tom