Fixed gcc warnings generated while linking against the library.

I was trying to link this against another project, and gcc threw a bunch of warnings due to definition/declaration issues. These changes take care of the issues, I think. The first extern is pretty straightforward. The fix for armv7_format_strings might seem slightly roundabout. ( The other way to fix that would be to do individual assignments, as in armv7_format_strings[I_ADC][0] = {"scdnmS", "scdni"} etc.)