build: Sync Gnulib scripts

* build-aux/git-log-fix: Updated to latest version.
* build-aux/git-version-gen: Likewise.
* build-aux/valgrind-tests.m4: Likewise.
build: Add contrib subdir with debian packaging stuff

* contrib/debian/changelog: New file; debian packaging stuff.
* contrib/debian/compat: Likewise.
* contrib/debian/control: Likewise.
* contrib/debian/copyright: Likewise.
* contrib/debian/rules: Likewise.
* contrib/debian/source/format: Likewise.
* maint.mk (debuild): New rule; build debian packages.