sync code with last improvements from OpenBSD

lib/mesa/include/no_extern_c.h

@@ -0,0 +1,48 @@
+/**************************************************************************
+ *
+ * Copyright 2014 VMware, Inc.
+ * All Rights Reserved.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included
+ * in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+ * OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.  IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR
+ * OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE,
+ * ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+ * OTHER DEALINGS IN THE SOFTWARE.
+ *
+ **************************************************************************/
+
+
+/*
+ * Including system's headers inside `extern "C" { ... }` is not safe, as system
+ * headers may have C++ code in them, and C++ code inside extern "C"
+ * leads to syntactically incorrect code.
+ *
+ * This is because putting code inside extern "C" won't make __cplusplus define
+ * go away, that is, the system header being included thinks is free to use C++
+ * as it sees fits.
+ *
+ * Including non-system headers inside extern "C"  is not safe either, because
+ * non-system headers end up including system headers, hence fall in the above
+ * case too.
+ *
+ * Conclusion, includes inside extern "C" is simply not portable.
+ *
+ *
+ * This header helps surface these issues.
+ */
+
+#ifdef __cplusplus
+template<class T> class _IncludeInsideExternCNotPortable;
+#endif