message: Make ./configure string consistent
  Summary:
  This was forgotten in some PR rebase.
  This is a backport of libsecp256k1 PR661
  Test Plan:
  ninja check-secp256k1
  Reviewers: #bitcoin_abc, nakihito
diff: @@ -145,7 +145,7 @@ AC_ARG_ENABLE(module_schnorr,
  [enable_module_schnorr=yes])
  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])
  