header-vars: new variable $(am__running_with_option)

header-vars: new variable $(am__running_with_option)

This is a preparatory refactoring, needed by later patches.
No semantic change is intended.

* lib/am/header-vars.am (am__running_with_option): New, contains
shell code that determines whether the current make instance is
running with a given one-letter option (e.g., -k, -n) that takes
no argument.  Actually, the only supported option at the moment
is '-n' (support for '-k' will be added soon).
(am__make_dryrun): Rewrite as a thin wrapper around
'$(am__make_running_with_option)'.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>