build-sys: First pass at a meson-ified build system

build-sys: First pass at a meson-ified build system

This is a working implementation of a build with meson. The server,
utils, and most modules build with this, and it is possible to run from
a build tree and play/capture audio on ALSA devices.

There are a number of FIXMEs, of course, and a number of features that
need to be enabled (modules, dependencies, installation, etc.), but this
should provide everything we need to get there relatively quickly.

To use this, install meson (distro package, or mesonbuild.com) and run:

  $ cd <pulseaudio src dir>
  $ meson <builddir>
  $ ninja -C <builddir>