The 'syntax-check' rule validates various coding style requirements.
It is run as part of the regular automated builds

* GNUmakefile, maint.mk: syntax-check rules (from GNULIB)
* cfg.mk: Local extra rules & exclusions
* build-aux/useless-if-before-free,
  build-aux/vc-list-files: Helpers for syntax check (from
  GNULIB)
* autobuild.sh: Run syntax-check as part of build
* configure.ac: Ensure GNUmakefile exists in VPATH builds