meson: use install_symlink() where applicable

Now our baseline of meson is 0.62, hence install_symlink() can be used.

Note, install_symlink() implies install_emptydir() for specified
install_dir. Hence, this also drops several unnecessary
install_emptydir() calls.

Note, the function currently does not support 'relative' and 'force' flags,
so several 'ln -frsT' inline calls cannot be replaced.

src/userdb/meson.build

@@ -34,9 +34,18 @@ if conf.get('ENABLE_SSH_USERDB_CONFIG') == 1
                 install_dir : sshdconfdir.startswith('/usr/') ? sshdconfdir : libexecdir / 'sshd_config.d')
 
         if conf.get('LINK_SSHD_USERDB_DROPIN') == 1
-                install_emptydir(sshdconfdir)
+                if meson.version().version_compare('>=1.3.0')
+                        install_symlink(
+                                '20-systemd-userdb.conf',
+                                pointing_to : fs.relative_to(libexecdir / 'sshd_config.d' / '20-systemd-userdb.conf',
+                                                             sshdconfdir),
+                                install_dir : sshdconfdir)
+                else
+                        install_emptydir(sshdconfdir)
 
-                meson.add_install_script(sh, '-c',
-                        ln_s.format(libexecdir / 'sshd_config.d' / '20-systemd-userdb.conf', sshdconfdir / '20-systemd-userdb.conf'))
+                        meson.add_install_script(sh, '-c',
+                                ln_s.format(libexecdir / 'sshd_config.d' / '20-systemd-userdb.conf',
+                                            sshdconfdir / '20-systemd-userdb.conf'))
+                endif
         endif
 endif