Make it possible to compile dump1090 without librtlsdr

[algefaen]

I needed to compile dump1090 to run as a "network hub" on a box where I don't have admin access, so I couldn't install librtlsdr. Solution: Add a Makefile option to disable all calls to rtlsdr. Cons: Adds #ifdefs to code, some may not like that.

Not tested very well yet. (It compiles and runs with and without this options, but haven't tested it with any data yet.)