Set noctilux-broken-srgb correctly in Emacs 24.4 on OS X

The new variable `ns-use-srgb-colorspace` is set to `t` by default,
so we check that too (when available) before concluding that the
colors are not sRGB.
Merge pull request #6 from purcell/patch-2

Set noctilux-broken-srgb correctly in Emacs 24.4 on OS X