Changing to latest release numbering: v2.4.18-beta

Moving the ConfigWindow display to Fl_Double_Window from Fl_Cairo_Win…

…dow which has issues redrawing the widgets on Big Sur

Updating the version string to the next release (v2.4.16-stable)

Updating the sources to work with the new FLTK 1.3.8 library and on B…

…ig Sur

Fixing -no-lto options on MacOS

Fixing compilation issues on MacOS with g++

Adding options to g++ to tell it to ignore unknown options II

Adding options to g++ to tell it to ignore unknown options II

Updating the build version

Updating release version -> v2.4.1-testing (II)