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.

configure.ac

@@ -174,6 +174,9 @@ AC_DEFINE_UNQUOTED([CPU_SPINWAIT], [$CPU_SPINWAIT])
 LD_PRELOAD_VAR="LD_PRELOAD"
 so="so"
 
+dnl Heap profiling uses the log(3) function.
+LIBS="$LIBS -lm"
+
 dnl Platform-specific settings.  abi and RPATH can probably be determined
 dnl programmatically, but doing so is error-prone, which makes it generally
 dnl not worth the trouble.
@@ -553,7 +556,6 @@ fi
 AC_MSG_CHECKING([configured backtracing method])
 AC_MSG_RESULT([$backtrace_method])
 if test "x$enable_prof" = "x1" ; then
-  LIBS="$LIBS -lm"
   AC_DEFINE([JEMALLOC_PROF], [ ])
 fi
 AC_SUBST([enable_prof])