Created attachment 885158 [details, diff] Single line patch 'panel-run-dialog.c' imports <gmenu-tree.h>. The '-I' flag for the compiler for gmenu3 isn't set unless $MENU_CFLAGS is added to org_gnome_gnome_panel_action_button_la_CFLAGS in the Makefile.in. After doing this the package builds normally. One line patch attached.
What package? Also, please include the full build.log & emerge --info. It's useful when sending it upstream to create a proper commit message.
Sorry, It is 'gnome-base/gnome-panel-3.50.0'.
Created attachment 885170 [details] Build log.
Created attachment 885171 [details] emerge --info