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/chunk.h

@@ -28,20 +28,14 @@
 #ifdef JEMALLOC_H_EXTERNS
 
 extern size_t		opt_lg_chunk;
-#ifdef JEMALLOC_SWAP
 extern bool		opt_overcommit;
-#endif
 
-#if (defined(JEMALLOC_STATS) || defined(JEMALLOC_PROF))
 /* Protects stats_chunks; currently not used for any other purpose. */
 extern malloc_mutex_t	chunks_mtx;
 /* Chunk statistics. */
 extern chunk_stats_t	stats_chunks;
-#endif
 
-#ifdef JEMALLOC_IVSALLOC
 extern rtree_t		*chunks_rtree;
-#endif
 
 extern size_t		chunksize;
 extern size_t		chunksize_mask; /* (chunksize - 1). */