build: add meson build system

build: add meson build system

Meson is easier to maintain, much faster, encourages better practices,
and is not built on a pile of shell scripts.

The autotools build system is kept intact for now, in order to ease the
migration. The intention is to remove it sooner rather than later, if
all goes well.

Run `meson build && mesonconf build` to see the configuration options
for the new system. Conversion should be straightforward. Environment
variables like CFLAGS work the same.

If meson is used, xorg-util-macros is not required.

In terms of functionality the two systems have about the same
capabilities. Here are some differences I noticed:

- Meson uses `-g` by default, autotools uses `-g -O2`.

- In autotools the default behavior is to install both static and shared
  versions of the libraries. In meson the user must choose exactly one
  (using -Ddefault_library=static/shared).

  It is possible to workaround if needed (install twice...), but
  hopefully meson will add the option in the future.

- Autotools has builtin ctags/cscope targets, meson doesn't.

  Easy to run the tools directly.

- Meson has builtin benchmarks target. Handy.

- Meson has builtin support for sanitizers/clang-analyzer/lto/pgo/
  coverage etc. Also handy.

Signed-off-by: Ran Benita <ran234@gmail.com>