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.

INSTALL

@@ -92,6 +92,7 @@ any of the following arguments (not a definitive list) to 'configure':
 --enable-debug
     Enable assertions and validation code.  This incurs a substantial
     performance hit, but is very useful during application development.
+    Implies --enable-ivsalloc.
 
 --enable-code-coverage
     Enable code coverage support, for use during jemalloc test development.
@@ -110,6 +111,11 @@ any of the following arguments (not a definitive list) to 'configure':
     Disable statistics gathering functionality.  See the "opt.stats_print"
     option documentation for usage details.
 
+--enable-ivsalloc
+    Enable validation code, which verifies that pointers reside within
+    jemalloc-owned chunks before dereferencing them.  This incurs a minor
+    performance hit.
+
 --enable-prof
     Enable heap profiling and leak detection functionality.  See the "opt.prof"
     option documentation for usage details.  When enabled, there are several