Add autoconf option to enable memory leak detection in libraries

There's a known memory leak in the engine_pkcs11 at the time of writing
this and it interferes with the named ability to check for memory leaks
in the OpenSSL memory context by default.

Add an autoconf option to explicitly enable the memory leak detection,
and use it in the CI except for pkcs11 enabled builds.  When this gets
fixed in the engine_pkc11, the option can be enabled by default.

doc/dev/dev.md

@@ -580,6 +580,19 @@ loops.
 None of these allocation functions, including `isc_mempool_get()`, can
 fail.  If no memory is available for allocation, the program will abort.
 
+The memory context can be set to check if all memory allocated via the said
+memory context was freed before the memory context was destroyed by calling
+`isc_mem_checkdestroyed()`.  This could lead to false positives on abnormal
+shutdowns, so the checking is only enabled in `dig` and `named` applications on
+normal shutdown.
+
+The memory context are normally used only for internal allocations, but several
+external libraries allow replacing their allocators (namely libxml2, libuv and
+OpenSSL).  As there has been known memory leak in the OpenSSL when
+`engine_pkcs11` is loaded, memory checking at destroy is disabled by default in
+the memory contexts used for external libraries and it needs to be enabled with
+a `--enable-leak-detection` autoconf option.
+
 #### <a name="lists"></a>Lists
 
 A set of macros are provided for creating, modifying and iterating