I'm a little confused, FLTK is including a libdecor plugin?
Yes. The libdecor source code
is copied, partially but unchanged,
to the FLTK source tree in libdecor/
[What is not copied is source code for meson-based building wich FLTK doesn't use.]
If there is another one installed on the machine, will that one be used in preference to the fltk one?
See below how to use the system-level libdecor when it's present on the system.
When it's not present or when we don't want to use the system version of libdecor,
libdecor is searched in the FLTK source tree, so there's no ambiguity if another
libdecor source tree exists elsewhere. That code is compiled and included in libfltk.
If the system contains an installed libdecor, turn on option OPTION_USE_SYSTEM_LIBDECOR in
CMake, and all resulting FLTK apps will be dynamically linked to the system's libdecor shared lib.
Also we cannot just assume the plugin exists?
We could, but libdecor is available only in quite recent Linux distros, not yet in the current long term support Ubuntu
I believe. I concluded it would be unwise to do that at this time.
@Bill: Let me know if you'd prefer another approach.