[PATCH] Mac OS X support

Hi,

Redshift is awesome and I use it all the time on my personal Debian laptop. I also use a Mac sometimes for work and I wanted it running there.

I've written a patch to add a gamma backend for the Quartz Display Services API, which is attached and also available at
https://ldpreload.com/p/redshift-macosx.patch

The patch is against the 1.7 release tarball (I don't have bzr on my Mac). Apart from autotools, it doesn't require anything that's not on a standard Mac system.

The API is documented at
https://developer.apple.com/library/mac/documentation/GraphicsImaging/Reference/Quartz_Services_Ref/Reference/reference.html
(see the "Adjusting the Display Gamma" section)

A couple of notes:
- The API seems to reset color configuration when the process exits. This is pretty reasonable, honestly, but it does mean that one-shot mode doesn't work.
- I added a fallback dummy definition of PKG_CHECK_MODULES to configure.ac so that I don't need pkg-config and its aclocal module installed.
- The API takes floats, not uint16_ts. Apparently every other backend wants uint16_t.
- The 0-to-1 ramp does not seem to be the default gamma ramp -- my first attempt made the screen significantly brighter and I think slightly bluer. So I assume that the current gamma ramp corresponds to 6500 K, and multiply the retrieved gamma ramp (instead of a 0-to-1 ramp) by the coefficients provided to me. This seems to be the only way to avoid a discontinuity with the fade-in. I refactored a bit of code in colorramp.c to accomplish this.
- I haven't yet added configuration for multiple monitors or similar.
- I haven't added a menubar icon a la gtk-redshift.

Let me know what you think. I'll add multiple monitor configuration and rebase against bzr sometime soon.