I downloaded FLTK 1.3.2 and the instructions for xcode 4 say that is enough to copy the FLTK folder (inside ide/Xcode4/Project Templates) into /Developer/Library/Xcode/Project/Templates/Other. So I did, but it seems like things have changed from xcode 4 to xcode 6: now all project templates have a directory that ends with .xctemplate, and inside the folder there is a single .plist file. Instead the FLTK folder has another hierarchy:

Indeed it doesn't work: the project template doesn't appear on the list when I select "new project" in Xcode.