This widget was designed for my own selfishly specific purposes, which is the reason behind its odd limitations, namely:
o The widget's height it a fixed size (FL_MATTE_BUTTON_HEIGHT) o The widget can only be set to certain colors
The Fl_Matte_Button project was designed so as to either be merged in the FLTK 1.3.x main tree, or just installed as a subdirectory of your individual project. The latter is recommended.
Here's a snapshot of the current version's documentation.