multimedia/arcan: unbreak on FreeBSD >= 12.1 i386 (LLD by default)

ld: error: cannot preempt symbol: alcOpenDevice

defined in /usr/local/lib/libopenal.so
referenced by arcan_audio.c

CMakeFiles/arcan.dir/engine/arcan_audio.c.o:(arcan_audio_setup)

Reported by: pkg-fallout