[configure/airtunes] - fix the fallback to libshairport if libshairplay can't be found...

... as pointed out by FneuFneu the fallback was not working because of poissoned variable use.

This fixes the fallback by tracking USE_AIRTUNES and only at the end of testing changing the use_airtunes variable.

@FneuFneu - can you test please? (i tried all constellations on ubuntu 10.4.2 - worked for me with this pr).