include: Add helper header to help trap includes inside extern C.

This is just to help repro and fixing these issues with any C++ compiler --

Commiting this will of course wait until all issues are addressed.

$ scons src/glsl/
scons: Reading SConscript files ...
Checking for GCC ...  yes
Checking for Clang ...  no
Checking for X11 (x11 xext xdamage xfixes glproto >= 1.4.13)... yes
Checking for XCB (x11-xcb xcb-glx >= 1.8.1 xcb-dri2 >= 1.8)... yes
Checking for XF86VIDMODE (xxf86vm)... yes
Checking for DRM (libdrm >= 2.4.38)... yes
Checking for UDEV (libudev >= 151)... yes
warning: LLVM disabled: not building llvmpipe
scons: done reading SConscript files.
scons: Building targets ...
scons: building associated VariantDir targets: build/linux-x86_64-debug/glsl
  Compiling src/glsl/ast_array_index.cpp ...
  Compiling src/glsl/ast_expr.cpp ...
  Compiling src/glsl/ast_function.cpp ...
  Compiling src/glsl/ast_to_hir.cpp ...
  Compiling src/glsl/ast_type.cpp ...
  Compiling src/glsl/builtin_functions.cpp ...
In file included from include/c99_compat.h:28:0,
                 from src/mapi/u_compiler.h:4,
                 from src/mapi/u_thread.h:47,
                 from src/mapi/glapi/glapi.h:47,
                 from src/mesa/main/mtypes.h:42,
                 from src/mesa/main/errors.h:47,
                 from src/mesa/main/imports.h:41,
                 from src/mesa/main/core.h:44,
                 from src/glsl/builtin_functions.cpp:58:
include/no_extern_c.h:48:1: error: template with C linkage
 template<class T> class _IncludeInsideExternCNotPortable;
 ^
In file included from include/c99_compat.h:28:0,
                 from include/c11/threads.h:38,
                 from src/mapi/u_thread.h:49,
                 from src/mapi/glapi/glapi.h:47,
                 from src/mesa/main/mtypes.h:42,
                 from src/mesa/main/errors.h:47,
                 from src/mesa/main/imports.h:41,
                 from src/mesa/main/core.h:44,
                 from src/glsl/builtin_functions.cpp:58:
include/no_extern_c.h:48:1: error: template with C linkage
 template<class T> class _IncludeInsideExternCNotPortable;
 ^
  Compiling src/glsl/builtin_types.cpp ...
  Compiling src/glsl/builtin_variables.cpp ...
scons: *** [build/linux-x86_64-debug/glsl/builtin_functions.os] Error 1
scons: building terminated because of errors.

Reviewed-by: Mark Janes <mark.a.janes@intel.com>

include/c99_compat.h

@@ -25,6 +25,8 @@
  *
  **************************************************************************/
 
+#include "no_extern_c.h"
+
 #ifndef _C99_COMPAT_H_
 #define _C99_COMPAT_H_
 

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 syntatically 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

src/util/u_atomic.h

@@ -6,6 +6,8 @@
  *
  */
 
+#include "no_extern_c.h"
+
 #ifndef U_ATOMIC_H
 #define U_ATOMIC_H