extensions-tool: Move to a subproject

The gnome-extensions tool code is really independent from the rest of the
code base, and could be used either as part of the gnome-shell build or as
stand-alone project (for example for the extension-ci docker image).

We can actually support both cases by moving the code to a subproject.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/877

subprojects/extensions-tool/meson.build

@@ -0,0 +1,42 @@
+project('gnome-extensions-tool', 'c',
+  version: '3.35.1',
+  meson_version: '>= 0.47.0',
+  license: 'GPLv2+'
+)
+
+gio_req = '>= 2.56.0'
+
+gnome = import('gnome')
+i18n  = import('i18n')
+
+package_name = get_option('package_name')
+prefix = get_option('prefix')
+
+bindir = join_paths(prefix, get_option('bindir'))
+datadir = join_paths(prefix, get_option('datadir'))
+mandir = join_paths(prefix, get_option('mandir'))
+
+localedir = join_paths(datadir, 'locale')
+
+gio_dep = dependency('gio-2.0', version: gio_req)
+gio_unix_dep = dependency('gio-unix-2.0', version: gio_req)
+autoar_dep = dependency('gnome-autoar-0')
+json_dep = dependency('json-glib-1.0')
+
+cc = meson.get_compiler('c')
+
+bash_completion = dependency('bash-completion', required: false)
+
+subdir('src')
+
+if bash_completion.found()
+  install_data('completion/bash/gnome-extensions',
+    install_dir: bash_completion.get_pkgconfig_variable('completionsdir')
+  )
+endif
+
+if get_option('man')
+  a2x = find_program('a2x')
+
+  subdir('man')
+endif