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.