[configure/airtunes] - fixed typo
[configure/airtunes] - fix fallback to libshairport if libshairplay can't be found. Bail out if --enable-airtunes was given and none of both libs was found.
Merge pull request #3972 from Memphiz/fixshairportfallback

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