build: Fix doc build, restore A2X assignment for doc/Makefile

build: Fix doc build, restore A2X assignment for doc/Makefile

Commit 4f2813a313ae ("build: Include generated man pages in dist
tarball") skips AC_CHECK_PROG for A2X altogether if doc/nft.8 is
already present.

Now, starting from a clean situation, we can have this sequence:
  ./configure	# doc/nft.8 not there, A2X set in doc/Makefile
  make		# builds doc/nft.8
  ./configure	# doc/nft.8 is there, A2X left empty in doc/Makefile
  make clean	# removes doc/nft.8
  make

resulting in:

  [...]
    GEN      nft.8
  /bin/sh: -L: command not found
  make[2]: *** [Makefile:639: nft.8] Error 127

and the only way to get out of this is to issue ./configure again
after make clean, which is rather unexpected.

Instead of skipping AC_CHECK_PROG when doc/nft.8 is present, keep
it and simply avoid returning failure if a2x(1) is not available but
doc/nft.8 was built, so that A2X is properly set in doc/Makefile
whenever needed.

Fixes: 4f2813a313ae ("build: Include generated man pages in dist tarball")
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Acked-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>