build: Include generated man pages in dist tarball

build: Include generated man pages in dist tarball

Most projects ship pre-generated man pages in the distribution tarball
so that builders don't need the documentation tools installed, similar
to how bison-generated sources are included.

To do this, we conditionalize the presence check of a2x on whether nft.8
already exists in the source directory, as it would exist if included in
the distribution tarball.

Secondly, we move the 'if BUILD_MAN' conditional to around the man page
generation rules. This ensures that the man pages are unconditionally
installed. Also only add the man pages to CLEANFILES if their generation
is enabled.

Signed-off-by: Matt Turner <mattst88@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>