message: build: Fix a non-portable use of `test`
diff: @@ -1450,7 +1450,7 @@ if test "$use_external_signer" != "no"; then
  [have_boost_process="yes"],
  [have_boost_process="no"])
  AC_MSG_RESULT([$have_boost_process])
  - if test "$have_boost_process" == "yes"; then
  + if test "$have_boost_process" = "yes"; then
  use_external_signer="yes"
  AC_DEFINE([ENABLE_EXTERNAL_SIGNER], [1], [Define if external signer support is enabled])
  else
  