Ben Gamari authored Oct 24, 2019 and Marge Bot committed Oct 29, 2019

Previously `hadrian` would pass `-optc-Werror=unused-but-set-variable`
to all GHC invocations. This was a difference from the make build system
and cause the unregisterised build to fail as the C that GHC produces
contains many unused functions. Drop it from the GHC flags.

Note, however, that the flag is still present in
`Settings.Builders.Common.cWarnings` and therefore will still be applied
during compilation of C sources.