Override makefile precious variable

configure.ac has precious variables, which are supposed to be subbed-in when calling configure, e.g. configuring the location of the system Lua interpreter as a precious variable[2] :

configure.ac:
  ...
  dnl Make LUA a precious variable.
  AC_ARG_VAR([LUA], [The Lua interpreter, e.g. /usr/bin/lua5.1])

command line:

./configure LUA=lua-5.3.6/src/lua

OR

export LUA=lua-5.3.6/src/lua
./configure

However, I'd like to hard code the location of the Lua interpreter instead of having to ask users to add it as an environment variable every time. How can I hard code it into configure.ac? https://www.gnu.org/software/autoconf/manual/autoconf-2.69/html_node/Setting-Output-Variables.html