build: Add common CLEANFILES and DISTCLEANFILES to common-rules.mk.

By adding common CLEANFILES and DISTCLEANFILES variables to
common-rules.mk, we can remove these from most other Makefiles, and
also clean files more consistently.

Note that bin_PROGRAMS are already cleaned by 'make clean', so I
removed cases where these were unnecessarily added to CLEANFILES.

ruby/Makefile.am

@@ -34,7 +34,7 @@ EXTRA_DIST = \
 	t/tc_*.rb \
 	t/test_helper.rb
 
-CLEANFILES = \
+CLEANFILES += \
 	lib/*~ \
 	t/*~ \
 	ext/guestfs/*~ \

ruby/examples/Makefile.am

@@ -23,10 +23,6 @@ EXTRA_DIST = \
 	inspect_vm.rb \
 	guestfs-ruby.pod
 
-CLEANFILES = \
-	guestfs-ruby.3 \
-	stamp-guestfs-ruby.pod
-
 man_MANS = guestfs-ruby.3
 noinst_DATA = $(top_builddir)/website/guestfs-ruby.3.html