meson: replace some m4 templates with jinja2

m4 was nice in '85, but the syntax feels a bit dated. Since we use python for
meson, let's use a popular python templating engine to replace some m4 usage.

A little nicety is that typos are caught:

FAILED: sysusers.d/systemd-remote.conf
/usr/bin/meson --internal exe --capture sysusers.d/systemd-remote.conf -- /home/zbyszek/src/systemd/tools/meson-render-jinja2.py config.h ../sysusers.d/systemd-remote.conf.j2
Traceback (most recent call last):
  File "/home/zbyszek/src/systemd/tools/meson-render-jinja2.py", line 28, in <module>
    print(render(sys.argv[2], defines))
  File "/home/zbyszek/src/systemd/tools/meson-render-jinja2.py", line 24, in render
    return template.render(defines)
  File "/usr/lib/python3.9/site-packages/jinja2/environment.py", line 1090, in render
    self.environment.handle_exception()
  File "/usr/lib/python3.9/site-packages/jinja2/environment.py", line 832, in handle_exception
    reraise(*rewrite_traceback_stack(source=source))
  File "/usr/lib/python3.9/site-packages/jinja2/_compat.py", line 28, in reraise
    raise value.with_traceback(tb)
  File "<template>", line 8, in top-level template code
jinja2.exceptions.UndefinedError: 'HAVE_MICROHTTP' is undefined

This checking mirrors what 349cc4a507 did for C defines.

sysusers.d/basic.conf.in

@@ -9,7 +9,7 @@
 u root    0     "Super User" /root
 
 # The nobody user for NFS file systems
-u @NOBODY_USER_NAME@  65534 "Nobody"     -
+u {{NOBODY_USER_NAME}}  65534 "Nobody"     -
 
 # Administrator group: can *see* more than normal users
 g adm     -     -            -
@@ -19,7 +19,7 @@ g wheel   -     -            -
 
 # Access to certain kernel and userspace facilities
 g kmem    -     -            -
-g tty     @TTY_GID@     -            -
+g tty     {{TTY_GID}}     -            -
 g utmp    -     -            -
 
 # Hardware access groups
@@ -36,4 +36,4 @@ g tape    -     -            -
 g video   -     -            -
 
 # Default group for normal users
-g users   @USERS_GID@     -            -
+g users   {{USERS_GID}}     -            -

sysusers.d/meson.build

@@ -4,32 +4,19 @@ if enable_sysusers
         install_data('README', install_dir : sysusersdir)
 endif
 
-in_files = ['basic.conf']
+in_files = [['basic.conf',          enable_sysusers],
+            ['systemd.conf',        enable_sysusers],
+            ['systemd-remote.conf', enable_sysusers and
+                                    conf.get('ENABLE_REMOTE') == 1 and conf.get('HAVE_MICROHTTPD') == 1]]
 
-foreach file : in_files
-        gen = configure_file(
-                input : file + '.in',
-                output : file,
-                configuration : substs)
-        if enable_sysusers
-                install_data(gen,
-                             install_dir : sysusersdir)
-        endif
-endforeach
-
-m4_files = ['systemd.conf']
-
-if conf.get('ENABLE_REMOTE') == 1 and conf.get('HAVE_MICROHTTPD') == 1
-        m4_files += ['systemd-remote.conf']
-endif
-
-foreach file : m4_files
+foreach tuple : in_files
+        file = tuple[0]
         custom_target(
-                'sysusers.d_' + file,
-                input : file + '.m4',
+                file,
+                input : file + '.in',
                 output: file,
-                command : [meson_apply_m4, config_h, '@INPUT@'],
+                command : [meson_render_jinja2, config_h, '@INPUT@'],
                 capture : true,
-                install : enable_sysusers,
+                install : tuple[1],
                 install_dir : sysusersdir)
 endforeach

sysusers.d/systemd-remote.conf.in

@@ -5,6 +5,6 @@
 #  the Free Software Foundation; either version 2.1 of the License, or
 #  (at your option) any later version.
 
-m4_ifdef(`HAVE_MICROHTTPD',
+{% if HAVE_MICROHTTPD %}
 u systemd-journal-remote  - "systemd Journal Remote"
-)m4_dnl
+{% endif %}

sysusers.d/systemd.conf.in

@@ -6,18 +6,18 @@
 #  (at your option) any later version.
 
 g systemd-journal   - -
-m4_ifdef(`ENABLE_NETWORKD',
+{% if ENABLE_NETWORKD %}
 u systemd-network   - "systemd Network Management"
-)m4_dnl
-m4_ifdef(`ENABLE_OOMD',
+{% endif %}
+{% if ENABLE_OOMD %}
 u systemd-oom       - "systemd Userspace OOM Killer"
-)m4_dnl
-m4_ifdef(`ENABLE_RESOLVE',
+{% endif %}
+{% if ENABLE_RESOLVE %}
 u systemd-resolve   - "systemd Resolver"
-)m4_dnl
-m4_ifdef(`ENABLE_TIMESYNCD',
+{% endif %}
+{% if ENABLE_TIMESYNCD %}
 u systemd-timesync  - "systemd Time Synchronization"
-)m4_dnl
-m4_ifdef(`ENABLE_COREDUMP',
+{% endif %}
+{% if ENABLE_COREDUMP %}
 u systemd-coredump  - "systemd Core Dumper"
-)m4_dnl
+{% endif %}