build: Pre-generate man pages at dist time

The man pages don't change very often, but draw in both docbook and
asciidoc. The latter is particularly problematic, as some distros
still ship only a python2 version of the tool.

Address this by generating the man pages at dist time, and including
the result in the tarball.

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1553>

meson/generate-manpages.py

@@ -0,0 +1,24 @@
+#!/usr/bin/env python3
+
+import os
+from pathlib import PurePath
+from asciidocapi import AsciiDocAPI
+
+man_pages = [
+    'man/gnome-shell.1',
+    'subprojects/extensions-tool/man/gnome-extensions.1',
+]
+
+sourceroot = os.environ.get('MESON_SOURCE_ROOT')
+distroot = os.environ.get('MESON_DIST_ROOT')
+
+asciidoc = AsciiDocAPI()
+
+for man_page in man_pages:
+    page_path = PurePath(man_page)
+    src = PurePath(sourceroot, page_path.with_suffix('.txt'))
+    dst = PurePath(distroot, page_path)
+    stylesheet = src.with_name('stylesheet.xsl')
+
+    asciidoc.options('--xsl-file', os.fspath(stylesheet))
+    asciidoc.execute(os.fspath(src), outfile=os.fspath(dst))