Using meson.source_root() and meson.build_root() are deprectated in
meson-0.56. Using current_source_dir() or current_build_dir() (which
have been available in all Meson versions) would require manually
adding "../" in some places. Instead, use project_source_root() and
project_build_root() and require meson-0.56.
Signed-off-by: Allan McRae <allan@archlinux.org>