Linux: include system Mesa headers when using system Mesa.

third_party/mesa/include/osmesa.h

Index: third_party/mesa/include/osmesa.h
diff --git a/third_party/mesa/include/osmesa.h b/third_party/mesa/include/osmesa.h
new file mode 100644
index 0000000000000000000000000000000000000000..d727e9045276bc265b0dfd971560789881c03729
--- /dev/null
+++ b/third_party/mesa/include/osmesa.h
@@ -0,0 +1,18 @@
+// Copyright (c) 2012 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef THIRD_PARTY_MESA_INCLUDE_OSMESA_H_
+#define THIRD_PARTY_MESA_INCLUDE_OSMESA_H_
+#pragma once
+
+// This is a shim header to include the right mesa header.
+// Use this instead of referencing the mesa header directly.
+
+#if defined(USE_SYSTEM_MESA)
+#include <GL/osmesa.h>
+#else
+#include "third_party/mesa/MesaLib/include/GL/osmesa.h"
+#endif
+
+#endif  // THIRD_PARTY_MESA_INCLUDE_OSMESA_H_