Change Lua to use GUIX_LUA_PATH and GUIX_LUA_CPATH to construct the default
LUA_PATH and LUA_CPATH, instead of using hard-coded paths that Guix doesn't
populate.

These paths don't use Lua's usual '?' path wildcard, and thus are compatible
with Guix's search-paths mechanism.

This patch uses functions defined in lua-5.x-search-path-helpers.patch.

--- a/src/loadlib.c
+++ b/src/loadlib.c
@@ -687,6 +687,9 @@
 }
 
 
+#include "./guixpaths.c"
+
+
 LUAMOD_API int luaopen_package (lua_State *L) {
   /* create table CLIBS to keep track of loaded C libraries */
   luaL_getsubtable(L, LUA_REGISTRYINDEX, CLIBS);
@@ -702,10 +757,18 @@
   lua_setfield(L, -3, "loaders");  /* put it in field `loaders' */
 #endif
   lua_setfield(L, -2, "searchers");  /* put it in field 'searchers' */
+
+  /* Calculate default LUA_PATH and LUA_CPATH values from their
+     corresponding GUIX_ environment variables */
+  const char* default_path = guix_path(L); // push default_path
+  const char* default_cpath = guix_cpath(L); // push default_cpath
+  lua_pushvalue(L, -3); // copy the old head of the stack back to the top
   /* set field 'path' */
-  setpath(L, "path", LUA_PATHVERSION, LUA_PATH, LUA_PATH_DEFAULT);
+  setpath(L, "path", LUA_PATHVERSION, LUA_PATH, default_path);
   /* set field 'cpath' */
-  setpath(L, "cpath", LUA_CPATHVERSION, LUA_CPATH, LUA_CPATH_DEFAULT);
+  setpath(L, "cpath", LUA_CPATHVERSION, LUA_CPATH, default_cpath);
+  lua_pop(L, 3); // pop our three working values back off the stack
+
   /* store config information */
   lua_pushliteral(L, LUA_DIRSEP "\n" LUA_PATH_SEP "\n" LUA_PATH_MARK "\n"
                      LUA_EXEC_DIR "\n" LUA_IGMARK "\n");