Don't warn on `mhs-options` (MicroHs options)

[sol]

When developing a package that supports both MicroHs and GHC it is useful to specify both mhs-options and ghc-options, for each respective compiler.

mhs-options are interpreted by MicroCabal and are ignored by Cabal, which is the desired behavior.

However, cabal check currently emits a warning on mhs-options, which it should not.