Restore --enable-ivsalloc.

However, unlike before it was removed do not force --enable-ivsalloc
when Darwin zone allocator integration is enabled, since the zone
allocator code uses ivsalloc() regardless of whether
malloc_usable_size() and sallocx() do.

This resolves #211.

configure.ac

@@ -625,7 +625,8 @@ fi
 
 dnl Do not compile with debugging by default.
 AC_ARG_ENABLE([debug],
-  [AS_HELP_STRING([--enable-debug], [Build debugging code])],
+  [AS_HELP_STRING([--enable-debug],
+                  [Build debugging code (implies --enable-ivsalloc)])],
 [if test "x$enable_debug" = "xno" ; then
   enable_debug="0"
 else
@@ -637,8 +638,28 @@ fi
 if test "x$enable_debug" = "x1" ; then
   AC_DEFINE([JEMALLOC_DEBUG], [ ])
 fi
+if test "x$enable_debug" = "x1" ; then
+  AC_DEFINE([JEMALLOC_DEBUG], [ ])
+  enable_ivsalloc="1"
+fi
 AC_SUBST([enable_debug])
 
+dnl Do not validate pointers by default.
+AC_ARG_ENABLE([ivsalloc],
+  [AS_HELP_STRING([--enable-ivsalloc],
+                  [Validate pointers passed through the public API])],
+[if test "x$enable_ivsalloc" = "xno" ; then
+  enable_ivsalloc="0"
+else
+  enable_ivsalloc="1"
+fi
+],
+[enable_ivsalloc="0"]
+)
+if test "x$enable_ivsalloc" = "x1" ; then
+  AC_DEFINE([JEMALLOC_IVSALLOC], [ ])
+fi
+
 dnl Only optimize if not debugging.
 if test "x$enable_debug" = "x0" -a "x$no_CFLAGS" = "xyes" ; then
   dnl Make sure that an optimization flag was not specified in EXTRA_CFLAGS.