build: merge all syntax-check logic into one file

build: merge all syntax-check logic into one file

The gnulib syntax-check rules are spread across GNUmakefile, cfg.mk and
maint.mk. This made sense when we were getting two of the files from the
gnulib submodule. Now that we own all files though, we can at least
merge maint.mk and cfg.mk together. GNUmakefile can be eliminated when
we switch to meson.

Reviewed-by: Michal Privoznik <mprivozn@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>