diff --git a/src/configure.ac b/src/configure.ac
index c024f65..d13e378 100644
--- a/src/configure.ac
+++ b/src/configure.ac
@@ -2001,6 +2001,9 @@
 	  RUBY_CFLAGS="-DDYNAMIC_RUBY_DLL=\\\"$libruby_soname\\\" $RUBY_CFLAGS"
 	  RUBY_LIBS=
 	fi
+	if test "X$CLANG_VERSION" != "X" -a "$rubyversion" -ge 30; then
+	  RUBY_CFLAGS="$RUBY_CFLAGS -fdeclspec"
+	fi
       else
 	AC_MSG_RESULT(not found; disabling Ruby)
       fi