Add USE_CSTD=c99 to fix:
lightdm-gtk-greeter.c: In function 'start_authentication':
lightdm-gtk-greeter.c:2023: error: 'for' loop initial declaration used outside C99 mode
Happens when building with base GCC.
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=237626