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).