diff --git a/src/auto/configure b/src/auto/configure
index f2486e9..ebeed2c 100755
--- a/src/auto/configure
+++ b/src/auto/configure
@@ -7651,6 +7651,9 @@
 	  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
 	{ $as_echo "$as_me:${as_lineno-$LINENO}: result: not found; disabling Ruby" >&5
 $as_echo "not found; disabling Ruby" >&6; }