Define FL_OVERRIDE unconditionally as 'override'

FLTK 1.5 requires at least C++11, hence we can always use 'override'.

To do: the FL_OVERRIDE macro can be replaced by 'override' everywhere
in the FLTK code. There are more than 2400 occurrences and this
would be a large commit. Therefore it's not done right now...

Note: user code may continue to use FL_OVERRIDE.