Remove usage of meson.source_root() and meson.build_root()
Remove usage of meson.source_root() and meson.build_root()

These were deprecated in favor of meson.project_source/build_root() and
meson.global_source/build_root(). The context on when to pick what
depends on what you are trying to do.

GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview,GNOME/gtksourceview