Add option to always build fuzz binaries

Add option to always build fuzz binaries

Currently the fuzzer binaries are only built when someone requests a
fuzzer. This might cause us to inadvertently break fuzzing when changing
function signatures. It also deviates with the behaviour we had with
autotools, where the fuzz binaries were built with make test.

This commit splits the -Dfuzzing option into two: fuzzing, and
fuzzing-backend. The fuzzing option controls whether the fuzzing
binaries are built. The fuzzing-backend option controls which backend to
use, and defaults to none. If the value none is used the binaries are
built, but no backend is used or guaranteed, which means that the
binaries might be non-functional.