Context: https://github.com/mesonbuild/meson/issues/4717
Using a directory directory as input in a custom_target was deprecated and printed the message
"This will become a hard error in the future."
It did become a hard error now.

Fixes error
../src/xrt/targets/steamvr_drv/meson.build:52:0: ERROR: File resources does not exist.
on meson 0.61.0

Christoph Haag authored Jun 10, 2020 and Jakob Bornecrantz committed Nov 09, 2020

v3:
  targets: Add Monado-SteamVR driver target
  st/ovrd: Add OpenVR driver header
  build: Factor out sdl hack into lib_sdl2_hack and update steamvr build
  build: Revert lib_sdl2_refactor
  steamvr: Emulate Index Controller by default
  steamvr: Use oxr_handle_destroy instead of exposing oxr_instance_destroy
  steamvr: don't use oxr internals
  steamvr: communicate 3dof tracking to steamvr
  steamvr: use util functions for device assignment and tracking origin setup
  steamvr: Install plugin to <prefix>/share/steamvr-monado
  steamvr: Use thread for updating poses every 1ms

Makes a big difference for the Index @144Hz on the vive driver.
Still somewhat choppy on survive driver - prediction should solve it.

Main-author: Christoph Haag <christoph.haag@collabora.com>
Co-author: Jakob Bornecrantz <jakob@collabora.com>