cabal-install updates to handle configInstantiateWith.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

cabal-install/Distribution/Client/ProjectConfig/Legacy.hs

@@ -539,6 +539,7 @@ convertToLegacyAllPackageConfig
       configHcFlavor            = projectConfigHcFlavor,
       configHcPath              = projectConfigHcPath,
       configHcPkg               = projectConfigHcPkg,
+      configInstantiateWith     = mempty,
       configVanillaLib          = mempty,
       configProfLib             = mempty,
       configSharedLib           = mempty,
@@ -604,6 +605,7 @@ convertToLegacyPerPackageConfig PackageConfig {..} =
       configHcFlavor            = mempty,
       configHcPath              = mempty,
       configHcPkg               = mempty,
+      configInstantiateWith     = mempty,
       configVanillaLib          = packageConfigVanillaLib,
       configProfLib             = packageConfigProfLib,
       configSharedLib           = packageConfigSharedLib,