Don't give warnings in compat
There are lots of warnings in here due to things like modules being imported that, in some versions of GHC, aren't used. Thus we don't give any warnings in here, and therefore validating with -Werror won't make the build fail. An alternative would be to do SRC_HC_OPTS := $(filter-out -Werror,$(SRC_HC_OPTS)) but if warnings are expected then there is little point in spewing them out anyway. On the other hand, there aren't any warnings for me (GHC 6.6 on Linux/amd64), so perhaps it would be worth fixing them instead.