build: Fix for failing 'make uninstall'

build: Fix for failing 'make uninstall'

Support for uninstalling is severely broken:

- extensions/GNUmakefile.in defines an 'install' target but lacks a
  respective 'uninstall' one, causing 'make uninstall' abort with an
  error message.

- iptables/Makefile.am defines an 'install-exec-hook' to create the
  binary symlinks which are left in place after 'make uninstall'.

Fix these problems by defining respective targets containing code copied
from automake-generated uninstall targets.

While being at it, add a few more uninstall-hooks removing custom
directories created by 'make install' if they are empty afterwards.

Reported-by: Richard Guy Briggs <rgb@redhat.com>
Signed-off-by: Phil Sutter <phil@nwl.cc>
Tested-by: Richard Guy Briggs <rgb@redhat.com>