Use systemd-tmpfiles for var dirs when on systemd

Putting packages files in /var prevent makes new install start with an
already populated /var, which prevent taking advantage of the systemd
provided tmpfiles.d/var.conf, which notably has this line:
q /var 0755 - - -
-> on supported filesystems (currently that means btrfs), this creates
/var as a subvolume, which has the benefit of excluding it from /
snapshots.

On non-systemd system, the `meson install` behavior stays the same,
/var/lib/pacman and /var/cache/pacman/pkg are included in DESTDIR.

Signed-off-by: Max Gautier <mg@max.gautier.name>

meson.build

@@ -445,9 +445,20 @@ install_data(
   'proto/proto.install',
   install_dir : join_paths(DATAROOTDIR, 'pacman'))
 
-foreach path : [
-	join_paths(LOCALSTATEDIR, 'lib/pacman/'),
-	join_paths(LOCALSTATEDIR, 'cache/pacman/pkg/'),
+systemd = dependency('systemd', required: false)
+if systemd.found()
+	install_data('tmpfiles.conf',
+				rename: 'pacman.conf',
+				install_dir: systemd.get_variable(pkgconfig: 'tmpfilesdir'))
+	var_paths = []
+else
+	var_paths = [
+		join_paths(LOCALSTATEDIR, 'lib/pacman/'),
+		join_paths(LOCALSTATEDIR, 'cache/pacman/pkg/'),
+		]
+endif
+
+foreach path : var_paths + [
 	join_paths(DATAROOTDIR, 'makepkg-template/'),
 	join_paths(DATAROOTDIR, 'libalpm/hooks/'),
 	]

tmpfiles.conf

@@ -0,0 +1,4 @@
+# Setup pacman data and cache directories
+
+d %S/pacman
+d %C/pacman/pkg