ghc-cabal: accept EXTRA_HC_OPTS make variable
ghc-cabal: accept EXTRA_HC_OPTS make variable

Signed-off-by: Sergei Trofimovich <df7d2cee2e7ececeb2368216a0f9d0537fd32149@google.com>

shlevy/ghc,snoyberg/ghc,ezyang/ghc,snoyberg/ghc,olsner/ghc,olsner/ghc,shlevy/ghc,mettekou/ghc,snoyberg/ghc,shlevy/ghc,sdiehl/ghc,snoyberg/ghc,ezyang/ghc,olsner/ghc,mettekou/ghc,olsner/ghc,mettekou/ghc,ezyang/ghc,ezyang/ghc,mettekou/ghc,shlevy/ghc,sdiehl/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc,mettekou/ghc,snoyberg/ghc,sdiehl/ghc,sdiehl/ghc,snoyberg/ghc,mettekou/ghc,olsner/ghc,sdiehl/ghc,shlevy/ghc,snoyberg/ghc,sdiehl/ghc,olsner/ghc,shlevy/ghc,olsner/ghc,ezyang/ghc,mettekou/ghc,ezyang/ghc