ut compilation fails on freebsd

ut-basic and ut-thread are failing on compilcation. This is on freebsd 11.3.

logs:

• https://jenkins.isc.org/job/kea-dev/job/ut-basic/190/execution/node/181/log/
• https://jenkins.isc.org/job/kea-dev/job/ut-thread/138/execution/node/38/log/

18:38:04.103  --- libkea_asiolink_la-process_spawn.lo ---
18:38:04.103  process_spawn.cc:265:13: error: use of undeclared identifier 'execvpe'; did you mean 'execve'?
18:38:04.103          if (execvpe(executable_.c_str(), args_.get(), vars_.get()) != 0) {
18:38:04.103              ^~~~~~~

the change that went into this build is #899 (closed)