Revert "Oops, libextl doesn't have build/libs.mk."
Revert "Oops, libextl doesn't have build/libs.mk."

.. but it does when building from inside the notion tree.

This reverts commit 4c0ad112333590ab74c0e21fdbcfa0f225ed2b31.

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