message: build: Make AM_CONDITIONAL([ENABLE_EXTERNAL_SIGNER]) unconditional
  This change fixes an error when the value of the "use_boost" variable is
  equal to "no".
diff: @@ -1416,8 +1416,6 @@ if test x$use_boost = xyes; then
  )
  fi
  - AM_CONDITIONAL([ENABLE_EXTERNAL_SIGNER], [test "x$use_external_signer" = "xyes"])
  -
  if test x$suppress_external_warnings != xno; then
  BOOST_CPPFLAGS=SUPPRESS_WARNINGS($BOOST_CPPFLAGS)
  fi
  @@ -1430,6 +1428,8 @@ if test x$use_boost = xyes; then
  BOOST_LIBS="$BOOST_LDFLAGS $BOOST_SYSTEM_LIB $BOOST_FILESYSTEM_LIB"
  fi
  +AM_CONDITIONAL([ENABLE_EXTERNAL_SIGNER], [test "x$use_external_signer" = "xyes"])
  +
  dnl Check for reduced exports
  if test x$use_reduce_exports = xyes; then
  AX_CHECK_COMPILE_FLAG([-fvisibility=hidden],[CXXFLAGS="$CXXFLAGS -fvisibility=hidden"],
  