Merge: frontend: make the warning on unavailable glslangValidator an advice

Changes the following warning to an advice and raise it at each use of the annotations (instead of only once).

~~~
/nit/lib/gamnit/flat.nit:355,9--26: Warning: program `glslangValidator` not in PATH, cannot validate this shader. (glslvalidator)
			""" @ glsl_vertex_shader
~~~

Pull-Request: #2299
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>