The problem is not gimp it is the the fact that it still uses lcms instead of lcms2. So just replace every occurence of lcms with lcms2 (should be 3 seperate.h and makefile) and recompile. I did not find any source or possibility to do a merge request. Maybe i will put it on my git repo. Or attach a patch here.
Please provide a build of Separate+ that works with GIMP 2.6.18.