Issue line number directives

[cjgillot]

Understanding compilation errors in the zpp output can be challenging because of mismatching line numbers.

C family languages support #line directives. Those can be used to re-sync the
compiler diagnostics to the pre-zpp input file.

I have a prototype implementation in gysela merge 633.

[jbigot]

Adding #line directive would indeed be a really good thing, I'd definitely accept a PR