Extend MAINTAINERCLEANFILES to include some things automatically added by libtoolize.

Makefile.am

@@ -156,4 +156,9 @@ MAINTAINERCLEANFILES =   \
   missing                \
   mkinstalldirs          \
   config.h.in            \
-  stamp.h.in
+  stamp.h.in             \
+  m4/ltsugar.m4          \
+  m4/libtool.m4          \
+  m4/ltversion.m4        \
+  m4/lt~obsolete.m4      \
+  m4/ltoptions.m4