YOU MUST CHECK OUT THE FLTK LIBS AND HEADERS

From your sbbs directory:
cvs -d :pserver:anonymous@cvs.synchro.net:/cvsroot/sbbs co lib/fltk/<os>
- AND -
cvs -d :pserver:anonymous@cvs.synchro.net:/cvsroot/sbbs co include

Where <os> is the lower case name of out OS... currently one of:
win32
linux
freebsd
openbsd