Automatically use LCMS if available
Fall back to __SSE__
Help when libpng is missing
Mark
honour option to disable cocoa

As the variable is set to 1 to enable and 0 to disable it will always
be defined.
Merge pull request #93 from keis/optional-cocoa

honour option to disable cocoa
Don't mark images.

This reverts commit 74e45a13bf54539442a3d2b5a35f3fc4685f18fa.
Avoid single colours to dominate the MSE calculation by limiting their perceptual weight to a maximum of 10% of the image surface.