use of `-fgen-c-line-directives` with versions >= 3.1

[GitMensch]

@OlegKunitsyn @marcsosduma

GnuCOBOL 3.1 added the option to generate line directives, so that GDB will actually know about the COBOL source lines.
This is auto-included -g. It may make the logic easier - and may also be used already, just wanted to drop a note about that.