Fix FreeBSD build errors

Tried building on FreeBSD and got these errors:

• backtrace could not be found, turns out you need to link with -lexecinfo: https://man.freebsd.org/cgi/man.cgi?query=backtrace&sektion=3&manpath=FreeBSD+11-current
• link errors caused by -lipx not existing (package no longer exist, so can no longer link against it)
• vmmeter no longer exists; it was never meant to be used directly but rather through sysctl

Changed the code to reflect these differences, and it now builds correctly.