Sorry, TL;DR the whole mail. Your problem is likely related due to absence of libGL due to global X11=off. gl.pc is libGL pkg-config file, and libGL requires X11 symbols to build. You likely do not need OPENGL and XCB for a headless setup, try to add the following to make.conf:
graphics_cairo_UNSET+= OPENGL XCB Also see bug 256180: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=256180 No idea if this mail will reach you (absence of AAAA RR for your MX), though desktop mailing list should CC you this mail.
