optionally use libngtcp2 only in development builds

optionally use libngtcp2 only in development builds

Unlike new transports with a new dependency DNS-over-QUIC support will
be added incrementally due to the non-trivial amound of plumbing
required by libngtcp2. This will require non-functional QUIC code in the
main branch that won't be exposed for non-development builds.

Therefore, libngtcp2 is linked as an optional dependency only on
explicitly enabled development builds and cannot be required. This will
be changed with a `doq` meson build option once the server-side
functionality is complete for consumption.