Allow for relative sysconfdir?

[flatsiedatsie]

It would be great if the directory where Shairport is looking for config files could be made relative, or changed on the fly. This would make it easier for 'portable' versions.

Currently when trying to build it with . as the config directory I get this warning:

configure: error: expected an absolute directory name for --sysconfdir: .

[flatsiedatsie]

I spoke before I thought. There is a -c configuration option already, allowing the specification of the config file path.