Revert "Insist that $(LUA) exists when building libextl-mkexports"
Revert "Insist that $(LUA) exists when building libextl-mkexports"

Breaks with dynamic values for $(LUA) :/

This reverts commit 2da1278d46f2a42b70ada5cc5ae914d094996545.

knixeur/notion,p5n/notion,anoduck/notion,knixeur/notion,raboof/notion,neg-serg/notion,neg-serg/notion,dkogan/notion,knixeur/notion,dkogan/notion,raboof/notion,p5n/notion,knixeur/notion,neg-serg/notion,anoduck/notion,dkogan/notion,raboof/notion,anoduck/notion,knixeur/notion,anoduck/notion,dkogan/notion,dkogan/notion.xfttest,p5n/notion,neg-serg/notion,anoduck/notion,p5n/notion,dkogan/notion,p5n/notion,dkogan/notion.xfttest,dkogan/notion.xfttest,raboof/notion,dkogan/notion.xfttest