mempool: only enable mempool use when linked to libsystemd-shared.so

Mempool use is enabled or disabled based on the mempool_use_allowed symbol that
is linked in.

Should fix assert crashes in external programs caused by #9792.
Replaces #10286.

v2:
- use two different source files instead of a gcc constructor

src/shared/enable-mempool.c

@@ -0,0 +1,5 @@
+/* SPDX-License-Identifier: LGPL-2.1+ */
+
+#include "mempool.h"
+
+const bool mempool_use_allowed = true;