classic: Pre-generate stylesheet

We follow the rule of not putting generated files under version
control, but that means drawing in additional build-time dependencies.
We can reduce those when building from a released tarball by
generating the stylesheets at dist time though, so do that.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/150>

data/meson.build

@@ -76,15 +76,20 @@ theme_data = [
   'gnome-classic-high-contrast.css'
 ]
 
-style = 'gnome-classic'
-custom_target(style + '.css',
-  input: style + '.scss',
-  output: style + '.css',
-  depend_files: theme_sources,
-  command: [sassc, '-a', '@INPUT@', '@OUTPUT@'],
-  install: true,
-  install_dir: themedir
-)
+stylesheet = 'gnome-classic.css'
+if fs.exists(stylesheet)
+  install_data(stylesheet, install_dir: themedir)
+else
+  sassc = find_program('sassc', required: true)
+  custom_target(stylesheet,
+    input: fs.replace_suffix(stylesheet, '.scss'),
+    output: stylesheet,
+    depend_files: theme_sources,
+    command: [sassc, '-a', '@INPUT@', '@OUTPUT@'],
+    install: true,
+    install_dir: themedir
+  )
+endif
 
 install_data(theme_data, install_dir: themedir)
 

meson.build

@@ -6,6 +6,7 @@ project('gnome-shell-extensions',
 
 gettext_domain = meson.project_name()
 
+fs = import('fs')
 gnome = import('gnome')
 i18n = import('i18n')
 
@@ -86,9 +87,10 @@ foreach e : enabled_extensions
 endforeach
 
 if classic_mode_enabled
-  sassc = find_program('sassc', required: true)
   subdir('data')
 endif
 
 subdir('extensions')
 subdir('po')
+
+meson.add_dist_script('meson/generate-stylesheets.py')

meson/generate-stylesheets.py

@@ -0,0 +1,13 @@
+#!/usr/bin/env python3
+
+import os
+from pathlib import PurePath
+import subprocess
+
+sourceroot = os.environ.get('MESON_SOURCE_ROOT')
+distroot = os.environ.get('MESON_DIST_ROOT')
+
+stylesheet_path = PurePath('data/gnome-classic.css')
+src = PurePath(sourceroot, stylesheet_path.with_suffix('.scss'))
+dst = PurePath(distroot, stylesheet_path)
+subprocess.call(['sassc', '-a', src, dst])