Add color management support via LittleCMS

At the moment this just assumes everything is sRGB, which works for the
vast majority of files in existence, but ideally we'd want to respect
tagged profiles in the file headers.

libexif and imlib2 don't seem to read them, though.
Add support for embedded EXIF and PNG ICC profiles
Add support for embedded JPEG ICC profiles

This adds all missing functionality w.r.t color management and therefore
resolves muennich/sxiv#189

I've also made the inclusion of colorspace. and iccjpeg.c conditional.