Makefile: remove duplication and refine the rule for considering README.rst out-of-date.
Makefile: remove duplication and refine the rule for considering README.rst out-of-date.

Signed-off-by: Daira Hopwood <cd7d43d5556a38ce626d53826fddc46adc597539@jacaranda.org>

zcash/zips,zcash/zips,zcash/zips,zcash/zips