The Fl_Matte_Button project can either be added to FLTK, or installed as a subdirectory either inside your own project, or in a separate directory that all your projects can refer to.
Just be sure to compile with the -I (include) flag pointing to where the Fl_Matte_Button directory is, and to link in the Fl_Matte_Button.o file left in that directory after doing a 'make'.
Here's an example of how to install Fl_Matte_Button into your project's own directory:
../src/YourApp/ | |-- YourApp.C |-- YourApp.H -- Contains "#include <FL/Fl_Matte_Button.H>" |-- Makefile -- Compiles with "-I./Fl_Matte_Button", links with ./Fl_Matte_Button/Fl_Matte_Button.o |-- Fl_Matte_Button -- Fl_Matte_Button installed as a subdirectory : | |-- FL | | | |-- Fl_Matte_Button.H | : | |-- Fl_Matte_Button.o |-- Makefile :This way you can still just have "#include <FL/Fl_Matte_Button.H>" in your C++ files.. just use "-I./Fl_Matte_Button" as one of the compile flags, and the #include will resolve correctly.
During linking, be sure to link in "./Fl_Matte_Button/Fl_Matte_Button.o" as part of the link instructions for your app.