build: DTrace is enabled by default on darwin

In configure, the --with-dtrace option only showed that it was true by
default on sunos. It is also true by default on darwin.

PR-URL: https://github.com/nodejs/io.js/pull/2019
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>

configure

@@ -230,7 +230,7 @@ parser.add_option('--with-mips-float-abi',
 parser.add_option('--with-dtrace',
     action='store_true',
     dest='with_dtrace',
-    help='build with DTrace (default is true on sunos)')
+    help='build with DTrace (default is true on sunos and darwin)')
 
 parser.add_option('--with-lttng',
     action='store_true',