make flags analysis: cater to GNU make 3.83 (still unreleased as of now)

make flags analysis: cater to GNU make 3.83 (still unreleased as of now)

The current development version of GNU make (that is planned to become
GNU make 3.83, sooner or later) has changed the format its $(MFLAGS)
variable slightly, removing the space between an option and its argument:

   # With GNU make 3.82, compiled from official tarball:
   $ make -f- <<<'all:; @echo "$$MFLAGS"' -I none
   -I none

   # With development version of GNU make (Git commit b5ea49b):
   $ make -f- <<<'all:; @echo "$$MFLAGS"' -I none
   -Inone

This was done on purpose, in order to support more easily the new
option '-O', which takes an optional argument; see:

  <http://lists.gnu.org/archive/html/bug-make/2013-05/msg00001.html>

So Just adapt to the new format as well.

* t/header-vars.am (am__make_running_with_option): Adjust to cater
to the new GNU make behaviour.

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