extension-tool/build: Install completions relative to our datadír

It is a good practice to install files relative to our own variables

https://www.bassi.io/articles/2018/03/15/pkg-config-and-paths/

and it is required on systems like NixOS.

Thanks to Iñigo, bash-completion support that since 2.10:

https://github.com/scop/bash-completion/pull/344

---

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/1194

subprojects/extensions-tool/meson.build

@@ -41,7 +41,7 @@ subdir('src')
 
 if bash_completion.found()
   install_data('completion/bash/gnome-extensions',
-    install_dir: bash_completion.get_pkgconfig_variable('completionsdir')
+    install_dir: bash_completion.get_pkgconfig_variable('completionsdir', define_variable: ['datadir', datadir])
   )
 endif