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