Fix memory tracking not being available on FreeBSD or macOS.

Closes #2096. Thanks to blusewang.

ChangeLog.txt

@@ -7,6 +7,7 @@ Broker:
   plugin should not be accessing them.
 - Give compile time warning if libwebsockets compiled without external poll
   support. Closes #2060.
+- Fix memory tracking not being available on FreeBSD or macOS. Closes #2096.
 
 Client library:
 - Fix mosquitto_{pub|sub}_topic_check() functions not returning MOSQ_ERR_INVAL

lib/memory_mosq.h

@@ -22,8 +22,10 @@ Contributors:
 #include <stdio.h>
 #include <sys/types.h>
 
-#if defined(WITH_MEMORY_TRACKING) && defined(WITH_BROKER) && defined(__GLIBC__)
-#define REAL_WITH_MEMORY_TRACKING
+#if defined(WITH_MEMORY_TRACKING) && defined(WITH_BROKER)
+#  if defined(__APPLE__) || defined(__FreeBSD__) || defined(__GLIBC__)
+#    define REAL_WITH_MEMORY_TRACKING
+#  endif
 #endif
 
 void *mosquitto__calloc(size_t nmemb, size_t size);