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

clang-7 is enable by default

Xenial is now available with Travis CI
https://blog.travis-ci.com/2018-11-08-xenial-release

clang-7 is enable by default

Xenial is now available with Travis CI
https://blog.travis-ci.com/2018-11-08-xenial-release

work with autotools and cmake

For GCC 5 and cmake 3.2.2, somehow the architecture is not extracted
from the implicit link information (see CMakeFiles/CMakeOutput.log).

Simplify PKG_CONFIG_PATH and LDFLAGS while at it using the new
--enable-asan option which automatically adds the required flags if
available. LDFLAGS is ignored by cmake, but apparently cmake builds
packages with the correct rpath without any further options.

Additionally, build examples by default (without -Werror) and remove
"cat config.log" which would result in configure succeeding while it
should not.

It fails for some reason.

git-subtree-dir: third-party/http-parser
git-subtree-split: 1b79abab34d4763c0467f1173a406ad2817c1635

This is probably perl which comes with precise is too old.  Upgrading
travis build to use trusty fixes the issue.

* Add cunit to run unit tests.
* Compile with more warning flags (--enable-werror)

Need to use clang-3.8 and gcc5 for c++std14