doc: Build documentation for functions even when they are unavailable on a particular platform due to lack of libraries. syscalls.cc (F_DUPFD, F_GETFD, F_GETFL, F_SETFD, F_SETFL, O_APPEND, O_ASYNC, O_CREAT, O_EXCL, O_NONBLOCK, O_RDONLY, O_RDWR, O_SYNC, O_TRUNC, O_WRONLY): Build documentation for functions even when they are not available due to lack of libraries. __init_fltk__.cc (Fgui_mode, Fmouse_wheel_zoom): Build documentation for functions even when they are not available due to lack of libraries.

gnulib = [git]git://git.sv.gnu.org/gnulib