simple fix for building flamerobin on Ubuntu 20.04 is :
sudo ln -s /bin/grep /usr/bin/grep
sudo ln -s /bin/egrep /usr/bin/egrep
tip via wxwidgets forum :
https://forums.wxwidgets.org/viewtopic.php?f=19&t=47403&p=200198#p200198
ps: on Ubuntu 22.04 wx-config works ok without the need for the above fix