make: Add target for building Teal sources

Compiled Lua targets included in parallel in order to not add a hard
dependency on Teal, at least not yet.

GNUmakefile

@@ -105,6 +105,10 @@ lint:
 	@echo $$(sed -n '/^\tlocal exclude_files/,/^}/p;' .luacheckrc | sed '1d;$d' | wc -l) files ignored
 	shellcheck configure
 
+util/%.lua: teal-src/util/%.tl
+	tl -I teal-src/ --gen-compat off --gen-target 5.1 gen $^ -o $@
+	-lua-format -i $@
+
 util/%.so:
 	$(MAKE) install -C util-src