Let plain ‘make’ work even not GNU Make

* Makefile.in (top_distclean): Clean makefile as well as Makefile.
* configure.ac: If not using plain ‘make’, create a makefile
so that plain ‘make’ simply calls $(MAKE).

Makefile.in

@@ -869,7 +869,7 @@ top_bootclean=\
 top_distclean=\
 	${top_bootclean}; \
 	rm -f config.status config.log~ \
-	  Makefile lib/gnulib.mk ${SUBDIR_MAKEFILES}
+	  Makefile makefile lib/gnulib.mk ${SUBDIR_MAKEFILES}
 
 distclean_dirs = $(clean_dirs) leim lisp
 

configure.ac

@@ -5814,4 +5814,12 @@ you can continue to support by using '$0 --with-pop'.])
   esac
 fi
 
-test "$MAKE" = make || AC_MSG_NOTICE([Now you can run '$MAKE'.])
+# Let plain 'make' work.
+test "$MAKE" = make || test -f makefile || cat >makefile <<EOF
+.POSIX:
+MAKE = $MAKE
+all:
+	\$(MAKE) -f Makefile \$?
+.DEFAULT:
+	\$(MAKE) -f Makefile \$<
+EOF