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The check-syntax target is enabled the use of Flymake's "legacy"
backend, flymake-proc-legacy-flymake in src/*.c.  It works quite well
with C sources.  The green light for this change had already been
given some time ago in

https://lists.gnu.org/archive/html/emacs-devel/2017-10/msg00385.html

* src/Makefile.in (check-syntax): New target.
(.PHONY): Add check-syntax.