protocol/Makefile: add `discard` target, and make the `linkcheck` target depend on `all-specs`.
protocol/Makefile: add `discard` target, and make the `linkcheck` target depend on `all-specs`.

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

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