Configure: enharden the check for luac 32bits

It could be a problem if one had no contribs, but /lib/liblua.a was
present

configure.ac

@@ -1551,7 +1551,7 @@ then
   AS_IF([test "${LUAC}" = "false"], [
     AC_MSG_ERROR([Could not find the LUA byte compiler.])
   ])
-  AS_IF([test -f ${CONTRIB_DIR}/lib/liblua.a -a `echo|${LUAC} -o - -|od -j 8 -N 2 -t x2|head -n 1|cut -d' ' -f2` != 0404], [
+  AS_IF([test -d ${CONTRIB_DIR} -a -f ${CONTRIB_DIR}/lib/liblua.a -a `echo|${LUAC} -o - -|od -j 8 -N 2 -t x2|head -n 1|cut -d' ' -f2` != 0404], [
     AC_MSG_ERROR([You need 32-bits luac when using using lua from contrib.])
   ])
 fi