Honour CC, CFLAGS and LDFLAGS in configure script.

This fixes feature detection when compiling w/o gcc (ie cross compiling).
Init scripts: improve documentation & fix paths.
tinysvcmdns: ensure name is advertised with .local suffix

Fixes operation with iOS 7.
Exit successfully with --help

Passing --help is a legitimate use, so it should not exit(1). Besides,
this is useful to test whether the command is available.
Clean up compile warnings and default to -Wall.

Hopefully this will make introducing new bugs more obvious.
Allow pipe output to run even if the reading end is not open (yet)
Add in launchd script for OS X.

Set up a daemon service for OS X using launchd that loads on boot.
The script depends on the computer having a network connection and
upon the mDNSResponder and coreaudiod running before it launches
shairport.

Other thoughts: Looking through the launchd options it would seem
that you could advertise the service using Bonjour via launchd
itself and then make it run only when a connection hits the port. I
think shairport would need to be changed so that it doesn't try and
advertise the service.