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.

Based on nghttp2 cmake build system. Advantage of this is a faster build
(cmake runs faster than configure and ninja is faster than make).

Changes from autotools:
* Warnings are enabled by default (independent of ENABLE_WERROR).
* ENABLE_WERROR has changed meaning (it just controls -Werror).
* include/ngtcp2/{version,ngtcp2}.h header files are installed.

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

Data received in Stream 0 STREAM frame in a protected packet is now
forwarded to TLS backend.  Those data received in Stream 0 STREAM
frame in a unprotected packet after handshake is ignored as before.

It fails for some reason.