Bug#1055502: liblcms2-2: Please build the fast-float and threaded plugins

[Adrian Knagg-Baugh]

Update: I hadn't considered that the main codebase is MIT licenced, so
the way to do this without mixing licences within a package would be
to generate a lcms2 package and a separate lcms2-plugins-gpl package,
each with their own -dev packages with the necessary header files.