build: Activate warnings and LTO on *BSD.

Newer versions of BSD ship compilers/packages which no longer
throw warnings, so activate the warnings on all platforms. Also LTO
(link-time optimization) seems to work on the test machines now,
so activate this as well.

Tested on FreeBSD-13.1 with clang-13.0.0 and on NetBSD-9.3 with
gcc-7.5.0.