#763 GTK3V2MAIN: Doxygen warning on older versions

[John R. Jackson]

The Doxygen PROJECT_ICON configuration option was added in version 1.10.0. The default Doxygen version installed on Ubuntu 24.04.4 LTS is 1.9.8 and doesn't have that option so Doxygen throws the following warning:

[ 97%] Building the XTrkCAD User Guide (Doxygen)
warning: ignoring unsupported tag 'PROJECT_ICON' at line 11, file work/XTrkCAD/GTK3V2MAIN/build/app/doc/Doxyfile

The attached patch comments out the PROJECT_ICON line from Doxyfile if the Doxygen version is less than 1.10.0.

[Allen D Byrne]

One thing to consider is should we consider a higher mininmum version for doxygen?
One should be able to install a newer version easily.
Otherwise no problem with the patch.