Fix luac detection error case

ac_cv_path_XX only caches positive results, not failures
(cherry picked from commit 6f81ceb1)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

configure.ac

@@ -1632,7 +1632,7 @@ then
   fi
   AC_ARG_VAR([LUAC], [LUA byte compiler])
   AC_PATH_PROGS(LUAC,[${LUAC} luac], [false])
-  AS_IF([test "${ac_cv_path_LUAC}" = "false"], [
+  AS_IF([test "${LUAC}" = "false"], [
     AC_MSG_ERROR([Could not find the LUA byte compiler.])
   ])
 fi