meson: correctly set NIX_CONF_DIR to $sysconfdir/nix

Instead of $sysconfdir.

Fixes #231, but there's more to do in following commits to make
Meson-built Lix actually look in /etc/nix.

Change-Id: Ia8d627070f405843add46e05cff5134b76b8eb48

src/libstore/meson.build

@@ -151,7 +151,7 @@ cpp_str_defines = {
   'NIX_DATA_DIR':  datadir,
   'NIX_STATE_DIR': state_dir / 'nix',
   'NIX_LOG_DIR':   log_dir,
-  'NIX_CONF_DIR':  sysconfdir,
+  'NIX_CONF_DIR':  sysconfdir / 'nix',
   'NIX_BIN_DIR':   bindir,
   'NIX_MAN_DIR':   mandir,
 }