Fix auto-enabling of Capsicum on FreeBSD with Autoconf. [skip appveyor]

Fix auto-enabling of Capsicum on FreeBSD with Autoconf. [skip appveyor]

Even after commit 6393bb6 --with-sandbox-capsicum didn't work entirely
as documented, as it defaulted to disabled:

checking whether to sandbox using capsicum... no
checking whether to sandbox using Casper library... no

Get the test condition right so it does what it says:

checking sys/capsicum.h usability... yes
checking sys/capsicum.h presence... yes
checking for sys/capsicum.h... yes
checking for cap_enter... yes
checking for cap_rights_limit... yes
checking for cap_ioctls_limit... yes
checking for openat... yes
checking for cap_init in -lcasper... yes
checking for cap_gethostbyaddr in -lcap_dns... yes
checking whether to sandbox using capsicum... yes
checking whether to sandbox using Casper library... yes