make flags analysis: be more robust

make flags analysis: be more robust

In particular, catering to option-with-argument bundled together
with options-without-arguments, as in:

   # With GNU make 3.82:
   $ make -f- <<<'all:; echo "$$MFLAGS"' -s -k -I none
   -skiI none

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

This fixes some lingering failures in the testsuite; precisely, in
tests 'make-dryrun.tap' and 'make-keepgoing.tap'.

* t/header-vars.am (am__make_running_with_option): Enhance.

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