Makefiles: change search through $(MAKEFLAGS) for GNU make 4.4

Since GNU make 4.4 the semantics of the $(MAKEFLAGS) variable has changed in a backward-incompatible way, as its "NEWS" file notes: Previously only simple (one-letter) options were added to the MAKEFLAGS variable that was visible while parsing makefiles. Now, all options are available in MAKEFLAGS. If you want to check MAKEFLAGS for a one-letter option, expanding "$(firstword -$(MAKEFLAGS))" is a reliable way to return the set of one-letter options which can be examined via findstring, etc. This upstream change meant that e.g.: make man Would become very noisy, because in shared.mak we rely on extracting "s" from the $(MAKEFLAGS), which now contains long options like "--jobserver-auth=fifo:<path>", which we'll conflate with the "-s" option. So, let's change this idiom we've been carrying since [1], [2] and [3] as the "NEWS" suggests. Note that the "-" in "-$(MAKEFLAGS)" is critical here, as the variable will always contain leading whitespace if there are no short options, but long options are present. Without it e.g. "make --debug=all" would yield "--debug=all" as the first word, but with it we'll get "-" as intended. Then "-s" for "-s", "-Bs" for "-s -B" etc. 1. 0c3b4aac8ec (git-gui: Support of "make -s" in: do not output anything of the build itself, 2007-03-07) 2. b777434383b (Support of "make -s": do not output anything of the build itself, 2007-03-07) 3. bb2300976ba (Documentation/Makefile: make most operations "quiet", 2009-03-27) Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>