The check for dtrace in configure.ac doesn't work on FreeBSD:
$DTRACE -G -o conftest.$$ -s memcached_dtrace.d
because the conftest file doesn't have any probes in it. The error message is:
dtrace: failed to link script memcached_dtrace.d: No probe sites found for declared provider
Also, it's good to pass -xnolibs to dtrace -G so that it doesn't processes the /usr/lib/dtrace scripts unnecessarily and then error out when it tries to talk to the DTrace kernel code, which happens when running in a jail currently. This isn't necessary on FreeBSD 11.0 or newer, but is a good idea until everyone has upgraded.
Not super sure how to fix? Want to give it a stab in a PR?
I still need to add a freebsd builder. should be in for the next major round, but right now it's still slightly difficult for me to validate/test stuff.