Use extension framework to enable #include directive.

This patch introduces a new extension, GL_GOOGLE_include_directive,
to enable support #include directives. It depends on the extension
GL_GOOGLE_cpp_style_line_directive.

Test/test-preprocessor-list

@@ -4,7 +4,8 @@ preprocessor.edge_cases.vert
 preprocessor.errors.vert
 preprocessor.extensions.vert
 preprocessor.function_macro.vert
-preprocessor.include.vert
+preprocessor.include.enabled.vert
+preprocessor.include.disabled.vert
 preprocessor.line.vert
 preprocessor.line.frag
 preprocessor.pragma.vert