Reduce cpp conditional logic complexity.

Convert configuration-related cpp conditional logic to use static
constant variables, e.g.:

  #ifdef JEMALLOC_DEBUG
    [...]
  #endif

becomes:

  if (config_debug) {
    [...]
  }

The advantage is clearer, more concise code.  The main disadvantage is
that data structures no longer have conditionally defined fields, so
they pay the cost of all fields regardless of whether they are used.  In
practice, this is only a minor concern; config_stats will go away in an
upcoming change, and config_prof is the only other major feature that
depends on more than a few special-purpose fields.

include/jemalloc/internal/ckh.h

@@ -30,10 +30,8 @@ struct ckhc_s {
 };
 
 struct ckh_s {
-#ifdef JEMALLOC_DEBUG
 #define	CKH_MAGIC	0x3af2489d
 	uint32_t	magic;
-#endif
 
 #ifdef CKH_COUNT
 	/* Counters used to get an idea of performance. */