[PATCH nft] build: Fix doc build, restore A2X assignment for doc/Makefile

[Stefano Brivio <sbrivio@redhat.com>]

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>
---
 configure.ac | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/configure.ac b/configure.ac
index 3496e410dbbe..5a1f89a0104c 100644
--- a/configure.ac
+++ b/configure.ac
@@ -50,9 +50,9 @@ AC_EXEEXT
 AC_DISABLE_STATIC
 CHECK_GCC_FVISIBILITY
 
-AS_IF([test "x$enable_man_doc" = "xyes" -a ! -f "${srcdir}/doc/nft.8"], [
+AS_IF([test "x$enable_man_doc" = "xyes"], [
        AC_CHECK_PROG(A2X, [a2x], [a2x], [no])
-       AS_IF([test "$A2X" = "no"],
+       AS_IF([test "$A2X" = "no" -a ! -f "${srcdir}/doc/nft.8"],
 	     [AC_MSG_ERROR([a2x not found, please install asciidoc])])
 ])
 
-- 
2.26.2