maint: avoid 'make syntax-check' from tarball

* .gnulib: update to latest gnulib for maint.mk fixes

diff --git a/.gnulib b/.gnulib
index ecb020f2b046790d76d05d377ca1e864e2bc9fdc..aa0f5d7586efe7044f6ca9e07be3f579ee0d5618 160000 (submodule)
--- a/.gnulib
+++ b/.gnulib
@@ -1 +1 @@
-Subproject commit ecb020f2b046790d76d05d377ca1e864e2bc9fdc
+Subproject commit aa0f5d7586efe7044f6ca9e07be3f579ee0d5618