BUILD: Makefile: have "make clean" destroy .o/.a/.s in contrib subdirs as well

Now that we sometimes link some contrib subparts directly into the
haproxy binary, it's becoming a real problem that they're not cleaned
on make clean.  Some of the tools there are useful as .so or pure
binaries and we don't want to remove them, but anything intermediary
susceptible to be linked into haproxy should be clenaed. This is what
this patch does for 3 levels of subdirs into contrib/, without touching
the rest. It should be sufficient for the vast majority of use cases.

(cherry picked from commit da867d8d68f19f9034a47d42d16db78a84a12299)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
diff --git a/Makefile b/Makefile
index 8b5f714..44cdfb8 100644
--- a/Makefile
+++ b/Makefile
@@ -961,6 +961,7 @@
 	$(Q)for dir in . src include/* doc; do rm -f $$dir/*~ $$dir/*.rej $$dir/core; done
 	$(Q)rm -f haproxy-$(VERSION).tar.gz haproxy-$(VERSION)$(SUBVERS)$(EXTRAVERSION).tar.gz
 	$(Q)rm -f haproxy-$(VERSION) haproxy-$(VERSION)$(SUBVERS)$(EXTRAVERSION) nohup.out gmon.out
+	$(Q)rm -f contrib/*/*.[oas] contrib/*/*/*.[oas] contrib/*/*/*/*.[oas]
 	$(Q)rm -f contrib/halog/halog contrib/debug/flags contrib/debug/poll contrib/tcploop/tcploop
 
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