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/preprocessor.include.disabled.vert

@@ -0,0 +1,7 @@
+#line 8000
+#include
+#include 123
+#include "foo"
+#include "foo" garbage
+#include "no-eol"
+