dry-run: with GNU make, prefer $(MFLAGS) over $(MAKEFLAGS)

dry-run: with GNU make, prefer $(MFLAGS) over $(MAKEFLAGS)

Fixes automake bug#13760 for GNU make.

* lib/am/header-vars.am (am__make_dryrun): If GNU make is being used, rely
on the contents of the $(MFLAGS) variable rather than of the $(MAKEFLAGS)
to decide whther make is being executed in "dry run" mode.  Not only this
makes the code possibly faster and less brittle, but also fixes automake
bug#13760 (at least when GNU make is in use).
* t/make-dryrun.tap: Adjust: some tests that were xfailing now pass.

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