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

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>