mesa: Add infrastructure for ARB_shading_language_420pack.

v2 [mattst88]
  - Split infrastructure into separate patch.
  - Add preprocessor #define.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>

src/glsl/glcpp/glcpp-parse.y

@@ -1242,6 +1242,9 @@ glcpp_parser_create (const struct gl_extensions *extensions, int api)
 
 	      if (extensions->AMD_vertex_shader_layer)
 	         add_builtin_define(parser, "GL_AMD_vertex_shader_layer", 1);
+
+	      if (extensions->ARB_shading_language_420pack)
+	         add_builtin_define(parser, "GL_ARB_shading_language_420pack", 1);
 	   }
 	}