Fix a dangerous bug in Makefile which actually allows 'make release' to skip

Fix a dangerous bug in Makefile which actually allows 'make release' to skip
building 'manpages' target because there is already 'manpages' directory and
some make versions are very sensible to what particular file system reports as
timestamp for the file or a directory.

In particular, this behaviour is reproduceable on some 2.4 kernels with Ext3.
Thanks to Dmitry Levin <ldv@altlinux.org> for spotting the bug and providing the
fix.
(This used to be commit 49afe716b080c08ecf5b2b00a89c9eee3a67ae4e)