[gfortran] Allow digits before X format to be optional

[Paul Brook]

> Will gfortran issue a warning (or similar message) that
> it is compiling non-conforming code?  Perhaps, we need to
> add a gfc_extension() to the gfc_error() and gfc_warning()
> functions.
>
> gfortran file.f
> Extension:  X edit descriptor used without integer field width.

We already have gfc_notify_std, which can be used to issue warnings/errors 
based on --std=<foo>.
Looking at the patch again, it should probably use this machanism.

Paul