Change to prevent the failure of "make distclean" on the missing 'regex/Makefile' (and, later, 'strings/Makefile').


configure.in:
  The subdirectories 'strings' and 'regex' were each contained in both the 'sql_client_dirs' and the 'sql_server_dirs' macro.
  As these are used to form the (Makefile) macro 'SUBDIRS' and 'DIST_SUBDIRS', these subdirectories will get listed there twice.
  This causes a failure of the (recursive) "make distclean", so this double inclusion must be prevented.