Add [make vendor].

Makefile

@@ -153,3 +153,16 @@ handiwork:
 	    opam install --yes zarith seq fix.20201120 ; \
 	  done ; \
 	  opam switch $$current
+
+# ------------------------------------------------------------------------------
+
+# Vendoring this library inside Menhir.
+
+.PHONY: vendor
+vendor:
+# Copy the library to Menhir's working directory.
+	@ make clean
+	@ make -f Makefile.vendor \
+	    THIS=feat \
+	    CLIENTS=$(HOME)/dev/menhir \
+	    SUPERFLUOUS=".git .gitignore headers Makefile Makefile.vendor README.md TODO.md" \