Meson is apparently able to show different warning depending on the
meson version tests we do in specific code paths.
Since our global required meson is 0.53.0 but on some code paths, we
test for 0.57.0, we must use meson.source_root() on older meson and
meson.project_source_root() on newer one.
Fixes:
> pdb/meson.build:141: WARNING: Project targets '>=0.57.0' but uses feature deprecated since '0.56.0': meson.source_root. use meson.project_source_root() or meson.global_source_root() instead.
> pdb/meson.build:141: WARNING: Project targets '>=0.57.0' but uses feature deprecated since '0.56.0': meson.build_root. use meson.project_build_root() or meson.global_build_root() instead.
> po-tips/meson.build:5: WARNING: Project targets '>=0.53.0' but uses feature introduced in '0.56.0': meson.project_source_root.
c857fc58a6