build: port to meson

doc/Doxyfile.meson.in

doc/meson.build

meson.build

meson_options.txt

src/CMakeLists.txt

src/external/meson.build

src/meson.build

src/targets_enabled_drivers.h.cmake_in

src/targets_enabled_drivers.h.meson_in

src/xrt/auxiliary/meson.build

src/xrt/compositor/meson.build

src/xrt/compositor/shaders/meson.build

src/xrt/drivers/meson.build

src/xrt/meson.build

src/xrt/state_trackers/meson.build

src/xrt/state_trackers/oxr/meson.build

src/xrt/state_trackers/prober/meson.build

src/xrt/state_trackers/prober/p_tracking.c

src/xrt/targets/CMakeLists.txt

src/xrt/targets/cli/meson.build