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.

[Bodigrim]

This would presumably require a new version of Cabal format?..

[geekosaur]

I don't see why, since we're not supporting it, only allowing other cabal implementations (specifically, MicroCabal) to support it.

[sol]

This would presumably require a new version of Cabal format?..

I don't see why, since we're not supporting it, only allowing other cabal implementations (specifically, MicroCabal) to support it.

That makes sense.

[sol]

It's a one line change (#11344). Reviews appreciated.