Merge bitcoin#661: Make ./configure string consistent

a467047 Make ./configure string consistent (Tim Ruffing)

Pull request description:

  This was forgotten in some PR rebase.

ACKs for top commit:
  jonasnick:
    ACK a467047

Tree-SHA512: 5aa67e886c165afa97a1e34ccfbd6bb0158ba4d4e5a4aacf6ac8b17ad9ee55132061957fd5ec383a79ad72ec7c92c745d7ad4fddca743b53e4b0e635616b29dc

configure.ac

@@ -135,7 +135,7 @@ AC_ARG_ENABLE(module_recovery,
     [enable_module_recovery=no])
 
 AC_ARG_ENABLE(external_default_callbacks,
-    AS_HELP_STRING([--enable-external-default-callbacks],[enable external default callback functions (default is no)]),
+    AS_HELP_STRING([--enable-external-default-callbacks],[enable external default callback functions [default=no]]),
     [use_external_default_callbacks=$enableval],
     [use_external_default_callbacks=no])