libnghttp2.so was missing -fvisibility=hidden. libnghttp2_asio.so on the
other hand had hidden visibility which resulted in no exported symbols
and a broken asio client examples.

Just build a static nghttp2 library to solve this issue.

Warnings are useful, let's enable them by default.

Some detected libraries were not reported when a feature is disabled.
This change removes unnecessary second find_package calls and sets
HAVE_xxx immediately based on xxx_FOUND.

Auto-detect spdylay availability using CMake, making pkg-config
completely optional.

FindCython.cmake was taken from
https://github.com/thewtex/cython-cmake-example. The UseCython module
works, but since it is lacking an installation target setup.py will be
used instead.

mruby is always invoked now (mirrors the autotools behavior). It could
be optimized though to only trigger the mruby build when the static
library is missing.

Also fix typo in NGHTTP2_TESTS_DIR macro definition (detected when
invoking the Ninja generator).

Need to add -fPIC to objects that will be put in a shared library.

Split the nghttp2 library into objects and a shared library from those
objects. This is needed because of symbol visibility. An advantage over
the autotools build is that there are no worries about static versus
static library builds.

Test:

    cmake $srcdir
    make nghttpx-unittest main failmalloc
    make test

Add auto-detection to the most important features (app, hpack, etc.).
Move options to a separate file for easier search.

Add cmake-based Libevent, jansson and CUnit search. Move pkg-config
handling for Libev and jemalloc to their cmake files.

Note: duplicates find_package before including CMakeOptions.txt and when
checking for features. Maybe that can be cleaned up later...

Remove build/host/target by a single target system name (CMake is
different for cross-compiling, you are suggested to set
CMAKE_TOOLCHAIN_FILE).

Fix various library variables, remove CFLAGS (INCLUDE_DIRS could be used
instead though, but I consider that minor information that could be
added later if wanted).

Fix various variable names (prefix, boost, etc.).

Might not be the best approach, maybe use thewtex/cython-cmake-example?

Avoids leaking compile flags to dependents.

Remaining work:
 - integrate mruby and neverbleed
 - integrate cunit

Must be absolute instead of relative

Do not use minus sign in macro names.

Also remove some headers which were not checked anyway and add macros to
cmakeconfig.h.in (based on the headers list in the CMakeLists.txt file).

AC_CHECK_TYPES and AC_C_BIGENDIAN were removed because nothing checks
the resulting macros...

Not working:
 - option(... check)
 - not finished everything (see XXX and FIXME)
 - still halway converting

`go test` requires both config.go and the test files in the same
directory. For out-of-tree builds, config.go is normally not placed next
to the source files, so copy the tests to the build directory as a
workaround.

Fixes multiple errors while making docs:

    Could not import extension sphinxcontrib.rubydomain (exception: No module named 'sphinxcontrib')

and

    ../../doc/sources/index.rst:15: WARNING: toctree contains reference to nonexisting document 'nghttp.1'
    ../../doc/sources/index.rst:15: WARNING: toctree contains reference to nonexisting document 'nghttpd.1'
    ../../doc/sources/index.rst:15: WARNING: toctree contains reference to nonexisting document 'nghttpx.1'
    ../../doc/sources/index.rst:15: WARNING: toctree contains reference to nonexisting document 'h2load.1'
    ../../doc/sources/index.rst:15: WARNING: toctree contains reference to nonexisting document 'programmers-guide'

Fixes the following two warnings:

    examples/client.c:292:0: error: macro "MAX_OUTLEN" is not used [-Werror=unused-macros]
    examples/tiny-nghttpd.c:298:13: error: function declaration isn’t a prototype [-Werror=strict-prototypes]

Caught using cmake as the autoconf check fails due to unused macros
(HAVE_xxx in conftest.c) and a main function without parameters
respectively.

Only used by lib/nghttp2_npn.c where the presence of config.h does not
seem to make a difference though.