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 means that $(MAKEFLAGS) now contains long options like "--jobserver-auth=fifo:<path>" and we have to adapt to that. Note that the "-" in "-$(MAKEFLAGS)" is critical here, as the variable will always contain leading whitespace if there are no short options, but long options are present. This is a partial backport of 67b36879fc0 (Makefiles: change search through $(MAKEFLAGS) for GNU make 4.4, 2022-11-30), which had been applied directly to git/git. Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com> Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Pratyush Yadav <me@yadavpratyush.com>