This adds a crypto backend based on GnuTLS.  While most of the
gnutls_* functions used in this backend are officially available in
upstream GnuTLS, the following functions are only available in the
'tmp-quic' branch, for ABI assurance reasons until the QUIC standard
is finalized:
- gnutls_handshake_write
- gnutls_quic_get_peer_transport_params

Signed-off-by: Daiki Ueno <dueno@redhat.com>
Signed-off-by: Anderson Toshiyuki Sasaki <ansasaki@redhat.com>

Require C++17 for structured binding.

Currently, when configuring with --disable-shared the following error is
generated when running make check:
make[2]: *** No rule to make target `../lib/.libs/*.o',
needed by `main'.  Stop.
make[2]: *** Waiting for unfinished jobs....

This commit adds a conditional for this case.

modify for FreeBSD. For using in6_addr structure in IPv6 protocl stack  KAME project add netinet/in.h header in util.cc . And for privent for conflict with bswap64 which defined in OS header, add #ifndef.

modify for FreeBSD. For using in6_addr structure in IPv6 protocl stack  KAME project add netinet/in.h header in util.cc . And for privent for conflict with bswap64 which defined in OS header, add #ifndef.

Enable warnings for C++ examples. Since there are some warnings, do not
disable -Werror for now.

As some of these are not supported in C++
Additionally, trim DEBUGFLAGS down to focus optimisation level
and debug symbol level only, as --enable-werror will enable
all of the warning flags that will ever be needed

- Define new CFLAGS in debug mode:
  - enable full embedding of debugging symbols
  - lower optimisation to avoid 'jumping' in gdb
  - enable stricter compiler warnings
- mitigate compiler warnings in client example
- Build client and server executables as binaries to
  allow them to be invoked directly by gdb