configure: Don’t error out when a2x doesn’t exist

The change in 2019-06-23T06:03:29Z!vilhelm.gray@gmail.com started
making it an error condition if a2x wasn’t found, which isn’t good.
Repair the operation so it works normally.

configure.ac

@@ -11,12 +11,7 @@ AC_ARG_ENABLE([man],
     AS_HELP_STRING([--disable-man],
                    [Disable manpage generation])
 )
-AS_IF([test "x$enable_man" != "xno"],
-    AS_IF([test "x$ASCIIDOC" != "xyes"],
-        AC_MSG_ERROR([AsciiDoc a2x program is required in order to build the manual])
-    )
-)
-AM_CONDITIONAL([ASCIIDOC], [test "x$enable_man" != "xno"])
+AM_CONDITIONAL([ASCIIDOC], [test "x$enable_man" != "xno" && test "x$ASCIIDOC" = "xyes"])
 
 AC_ARG_WITH([libpng],
     AS_HELP_STRING([--without-libpng],