Fix otherlibs/Makefile (follow-up to #11160).

In this file, the clean targets depend on the OTHERLIBRARIES variable.

However, this variable is defined in `Makefile.config`, which is not
included when make is called for cleaning, so that the clean does
actually not happen (OTHERLIBRARIES being empty in that case).

otherlibs/Makefile

@@ -16,6 +16,13 @@
 ROOTDIR=..
 include $(ROOTDIR)/Makefile.common
 
+# Also the OTHERLIBRARIES variable is defined in ../Makefile.config,
+#  its following conditional definition needs to be kept because,
+# at the moment, the clean targets depend on this variable but
+# when they are invoked ../Makefile.config is not included, so that
+# OTHERLIBRARIES would be empty and the clean targets would thus not work.
+OTHERLIBRARIES ?= dynlink str systhreads unix
+
 # $1: target name to dispatch to all otherlibs/*/Makefile
 define dispatch_
 $1: