freebsd: don't enable dtrace if it's not available
freebsd: don't enable dtrace if it's not available

unknownbrackets/maxcso,urbit/archaeology2,urbit/archaeology2,unknownbrackets/maxcso,urbit/archaeology2,urbit/archaeology2,unknownbrackets/maxcso,unknownbrackets/maxcso,urbit/archaeology2,urbit/archaeology2,unknownbrackets/maxcso,urbit/archaeology2,urbit/archaeology2,unknownbrackets/maxcso,unknownbrackets/maxcso,urbit/archaeology2