Use glibc allocator hooks.

When jemalloc is used as a libc malloc replacement (i.e. not prefixed),
some particular setups may end up inconsistently calling malloc from
libc and free from jemalloc, or the other way around.

glibc provides hooks to make its functions use alternative
implementations.  Use them.

Submitted by Karl Tomlinson and Mike Hommey.

src/mutex.c

@@ -1,6 +1,10 @@
 #define	JEMALLOC_MUTEX_C_
 #include "jemalloc/internal/jemalloc_internal.h"
 
+#ifdef JEMALLOC_LAZY_LOCK
+#include <dlfcn.h>
+#endif
+
 /******************************************************************************/
 /* Data. */