There seems to be the -o flag in several places:

./mswindows/osgeo4w/postinstall.bat ./mswindows/GRASS-Installer.nsi.tmpl ./lib/driver/font2.c ./binaryInstall.src

This should be probably replaced with --o, no?

Yes, better to do it now even though it's backward compatible.

Assuming there aren't missing places that also need this change, I agree with this!

I couldn't find any other places than renamed_options using

$ find -type f ! -name '*.o' ! -name '*.so' ! -name '*.0dev' -exec grep -H 'g.mkfontcap.*[^-]-o' {} \; |
  grep -Ev '^./((bin|dist).x86_64-pc-linux-gnu|.git|build)'
./lib/gis/renamed_options:g.mkfontcap|-o:--overwrite