There was some discussion on the FLTK mailing lists about different ways to implement dockable toolbars in a portable, cross-platform manner.
The following is my offering in that regard.
You will need a copy of FLTK built and installed.
Then you need this zip, which has the sources for the program.
Then type make and hope for the best!
This has been built, from this zip, on OSX, linux and Win32. And it works just fine.
On Win32 platforms it was built using the mingw and Msys tools, obtained from Sourceforge. These provide a free gcc derived toolchain for building native Win32 applications. And they work very well. It should work fine with any toolchain that supports fltk.
The code's a bit rough'n'ready, as it was only intended as a proof of concept. However, it works OK and I have used it in a few things. There's a bit of diagnostic chatter (printf's that is) from the code as it runs, so if you run it in a console you can see what it is doing. You'd want to take that out of a "real" version, of course!