Add Meson build

Meson is a next generation build system, simpler than Autotools and also faster
and more portable. Most importantly, it will make integrating ASan easier in
CI.

The goal is to maintain feature parity of the Meson build with the
Autotools build, until such time when we can drop the latter.

Add a script which generates the desired Doxygen configuration for our various
output formats and executes it using that configuration. This is not something
Meson can or should do.

Fixes: https://gitlab.freedesktop.org/wayland/wayland/issues/80



[daniels: Changed to bump version, use GitLab issues URL, remove header
          checks not used in any code, remove pre-pkg-config Expat
	  support, added missing include paths to wayland-egl and
	  cpp-compile-test, added GitLab CI.
	  Bumped version, removed unnecessary pkg-config paths.]

[daniels: Properly install into mandir/man3 via some gross
          paramaterisation, generate real stamp files.]

Pekka:
- squashed patches
- removed MAKEFLAGS from meson CI
- remove unused PACKAGE* defines
- fix up scanner dependency handling
- instead of host_scanner option, build wayland-scanner twice when cross-compiling
- changed .pc files to match more closely the autotools versions
- reorder doxygen man sources to reduce diff to autotools
- fix pkgconfig.generate syntax warnings (new in Meson)
- bump meson version to 0.47 for configure_file(copy) and run_command(check)
- move doc tool checks into doc/meson.build, needed in more places
- make all doc tools mandatory if building docs
- check dot and doxygen versions
- add build files under doc/publican
- reindent to match Weston Meson style

Simon:
- Remove install arg from configure_file
- Don't build wayland-scanner twice during cross-build
- Fix naming of the threads dependency
- Store tests in dict
- Add missing HAVE_* decls for functions
- Remove unused cc_native variable
- Make doxygen targets a dict
- Make dot_gv a dict
- Use dicts in man_pages
- Make decls use dicts
- Make generated_headers use dicts
- Align Meson version number with autotool's

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Signed-off-by: Simon Ser <contact@emersion.fr>