dry run: use $(MFLAGS), not $(MAKEFLAGS), to determine whether running dry

dry run: use $(MFLAGS), not $(MAKEFLAGS), to determine whether running dry

$(MFLAGS) does not containing the command line variable definitions, and
it always begins with a hyphen unless it is empty (MAKEFLAGS begins with
a hyphen only when it begins with an option that has no single-letter
version, such as '--warn-undefined-variables').  This allows use to make
stricter assumptions on its content, thus simplifying the implementation
of ...

* lib/am/header-vars.am (am__make_dryrun): ... this macro.
* t/make-dryrun.tap: Simplify accordingly.

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