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/basic/mempool.h

@@ -1,6 +1,7 @@
 /* SPDX-License-Identifier: LGPL-2.1+ */
 #pragma once
 
+#include <stdbool.h>
 #include <stddef.h>
 
 struct pool;
@@ -22,6 +23,7 @@ static struct mempool pool_name = { \
         .at_least = alloc_at_least, \
 }
 
+extern const bool mempool_use_allowed;
 bool mempool_enabled(void);
 
 #if VALGRIND