meson: add option to skip installing to $sysconfdir

This is useful for development where overwriting files out side
the configured prefix will affect the host as well as stateless
systems such as NixOS that don't let packages install to /etc but handle
configuration on their own.

Alternative to https://github.com/systemd/systemd/pull/17501

tested with:

$ mkdir inst build && cd build
$ meson \
  -Dcreate-log-dirs=false \
  -Dsysvrcnd-path=$(realpath ../inst)/etc/rc.d \
  -Dsysvinit-path=$(realpath ../inst)/etc/init.d \
  -Drootprefix=$(realpath ../inst) \
  -Dinstall-sysconfdir=false \
  --prefix=$(realpath ../inst) ..
$ ninja install

network/meson.build

@@ -10,8 +10,10 @@ if conf.get('ENABLE_NETWORKD') == 1
                      '80-wifi-station.network.example',
                      install_dir : networkdir)
 
-        meson.add_install_script('sh', '-c',
-                                 mkdir_p.format(join_paths(sysconfdir, 'systemd/network')))
+        if install_sysconfdir
+                meson.add_install_script('sh', '-c',
+                                         mkdir_p.format(join_paths(sysconfdir, 'systemd/network')))
+        endif
 endif
 
 install_data('99-default.link',