configure.ac: Require a2x when building from git

Copied over from libquvi {6cdebfd, 6612228}.

Signed-off-by: Toni Gundogdu <legatvs@gmail.com>
FIX: configure.ac: AS_IF for date(1)

Signed-off-by: Toni Gundogdu <legatvs@gmail.com>