Add CMake build system support

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.