Is there any way to define flags for cabal dependencies?

Newer versions of Cabal let you specify constraints in your cabal.project.local or cabal.project file. For example:

constraints: hmatrix +openblas

Is there any way to indicate in my application's .cabal file that I depend on a library so that it is built with the specific build flag?

No, but in your case this is not actually a problem in the solver and is rather and uninformative error (caused by someone's less than judicious uses of flags).

I also struggled for a long time to find a solution to this problem. I just found one! You have to modify the global cabal configuration file at ~/.cabal/config. Add a constraints line like this to the initial section of the file:

constraints: hmatrix +openblas

This enables the openblas flag for the hmatrix package. It will be used automatically the next time the package is installed. If there is a way to set such a flag locally for a sandbox, I could not find it.

Looks like it's not possible to specify such a dependency via the build-depends field in your .cabal file. buildDepends is defined as [Dependency], where data Dependency = Dependency PackageName VersionRange. You can use cabal install --constraint="transformers-compat +transformers3", though.

Looking at the transformers-compat.cabal file, I think that the solver should be able to figure out the correct flag assignment if you constrain your dependency on transformers appropriately. E.g. build-depends: transformers >= 0.3 && < 0.4 should force the solver to choose transformers-compat +transformers3. If this doesn't work, it may be a bug in the solver.