Clean crypto/Makefile and doc/Makefile on superclean.

Add distclean as a synonym for superclean.
diff --git a/Makefile.in b/Makefile.in
index de890d1..e88090a 100644
--- a/Makefile.in
+++ b/Makefile.in
@@ -186,7 +186,7 @@
 libsrtpdoc:
 	$(MAKE) -C doc
 
-.PHONY: clean superclean install
+.PHONY: clean superclean distclean install
 
 install:
 	@if [ -r $(DESTDIR)$(includedir)/srtp/srtp.h ]; then \
@@ -219,9 +219,12 @@
 
 superclean: clean
 	rm -rf crypto/include/config.h config.log config.cache config.status \
-               Makefile .gdb_history test/.gdb_history .DS_Store
+               Makefile crypto/Makefile doc/Makefile \
+               .gdb_history test/.gdb_history .DS_Store
 	rm -rf autom4te.cache
 
+distclean: superclean
+
 distname = srtp-$(shell cat VERSION)
 
 distribution: runtest superclean