Until mainline support of QUIC are present in OpenSSL and GnuTLS, be
sure to test whether the found library versions are the patched ones
since the version number check is insufficient. This fixes the default
build on a system where a new GnuTLS is present without the patches from
the GnuTLS tmp-quic branch.

Add new ENABLE_OPENSSL and ENABLE_GNUTLS CMake options to allow
explicitly disabling these libraries to mirror the --without-openssl and
the --without-gnutls functionality of autotools.

Partially addresses https://github.com/ngtcp2/ngtcp2/issues/238

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