configure: luac: autodetect host if not provided

When building the luac contrib, it is built at $(PREFIX)/../bin/$(cc -dumpmachine)-luac

When configuring, if --host is not provided, the script looks for luac at $(PREFIX)/../bin/-luac as $host_alias is empty, failing to lookup luac.

Use (cc -dumpmachine) to look for the LUA byte compiler when host is not provided.

configure.ac

@@ -592,8 +592,12 @@ AS_IF([test -n "${CONTRIB_DIR}"], [
 
   AS_IF([test -z "$LUAC"], [
     AC_MSG_CHECKING([if contribs provide luac])
-    AS_IF([test -x "${CONTRIB_DIR}/../bin/${host_alias}-luac${BUILDEXEEXT}"], [
-      LUAC="${CONTRIB_DIR}/../bin/${host_alias}-luac${BUILDEXEEXT}"
+    LUAC_HOST=${host_alias}
+    AS_IF([test -z "$LUAC_HOST"], [
+           LUAC_HOST="`$CC -dumpmachine`"
+    ])
+    AS_IF([test -x "${CONTRIB_DIR}/../bin/${LUAC_HOST}-luac${BUILDEXEEXT}"], [
+      LUAC="${CONTRIB_DIR}/../bin/${LUAC_HOST}-luac${BUILDEXEEXT}"
       AC_MSG_RESULT([yes])
     ],[
       AC_MSG_RESULT([no])