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.c

@@ -78,6 +78,8 @@ bool mempool_enabled(void) {
         if (!is_main_thread())
                 return false;
 
+        if (!mempool_use_allowed)
+                b = false;
         if (b < 0)
                 b = getenv_bool("SYSTEMD_MEMPOOL") != 0;