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)
mdns_external: don't warn loudly, these are often expected to fail

eg. on many avahi and all tinysvcmdns systems.
audio_pipe: don't use the name 'wait'

Fixes compile on systems where POSIX wait is in the namespace.
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.
rtsp: fix response header allocation in handle_setup

With thanks to @yenchee1970 for pointing out the bug.
debian scripts: set default group to audio

Closes #337.
Requesting progress and metadata in MDNS announcement.
Switching to sub handlers in handle_set_parameter by content-type.
Moved actual parameter handler.
Added empty handlers for cover art.
Added preliminary parsing code for DMAP tags.
Added metadata structures and methods.
Added command line option to set the cover art output directory.
Added player methods for printing metadata and setting the artwork image.
Added metadata.c to makefile.