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>