build: validate headers against local gnulib not git repo

Some syntax check rules validate usage of headers provided
by gnulib. We want to validate these only against the gnulib
modules we've chosen to use, not all modules, since we're
trying to eliminate them.
Reviewed-by: NPavel Hrdina <phrdina@redhat.com>
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

build-aux/syntax-check.mk

@@ -132,9 +132,6 @@ local-check :=								\
 
 syntax-check: $(local-check)
 
-# We use .gnulib, not gnulib.
-gnulib_dir = $(srcdir)/.gnulib
-
 # We haven't converted all scripts to using gnulib's init.sh yet.
 _test_script_regex = \<\(init\|test-lib\)\.sh\>