Ports with USES=localbase and lang/gcc* look under /usr/local/include by default. Unexpected consumers may auto-detect sysinfo() based on sys/sysinfo.h availability but assume the symbol is in libc, not a separate library. In shared libraries underlinking may go unnoticed.
Steps to reproduce:
1. pkg install gcc9 libsysinfo xorg-minimal
2. CC=gcc9 make install clean -C/usr/ports/x11-drivers/xf86-video-intel
3. Adjust xorg.conf to use "intel" driver
5. Livelock from VT switch to X server that failed to start
3.1. Add xorg.conf to use Option "AccelMethod" "SNA"