This allows you to say things like
    SRC_HC_WARNING_OPTS += -fno-warn-unsupported-calling-conventions
in mk/validate.mk.

Unfortunately, we can't just use SRC_HC_OPTS, as that gets put before
the more specific options (e.g. ghc-options in a .cabal file), many of
which include -Wall. So now we have:
    ghc $(SRC_HC_OPTS) ... options from .cabal etc ... $(SRC_HC_WARNING_OPTS)