--- a/build/linux/unbundle/openh264.gn
+++ b/build/linux/unbundle/openh264.gn
@@ -2,6 +2,7 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+import("//build/buildflag_header.gni")
 import("//build/config/linux/pkg_config.gni")
 import("//build/shim_headers.gni")
 
@@ -34,3 +35,9 @@ source_set("encoder") {
   deps = [ ":openh264_shim" ]
   public_configs = [ ":config" ]
 }
+
+buildflag_header("buildflags") {
+  header = "buildflags.h"
+  header_dir = "third_party/openh264"
+  flags = [ "OPENH264_API_WELS=1" ]
+}