New override of find_program('meson')

In some cases, it has been useful for build scripts to access the Meson command used to invoke the build script. This has led to various ad-hoc solutions that can be very brittle and project-specific.

meson_prog = find_program('meson')

This call will supply the build script with an external program pointing at the invoked Meson.

Because Meson also uses find_program for program lookups internally, this override will also be handled in cases similar to the following:

custom_target(
  # ...
  command: [
    'meson',
  ],
  # ...
)

run_command(
  'meson',
  # ...
)

run_target(
  'tgt',
  command: [
    'meson',
    # ...
  ]
)