Merge pull request #3924 from ezyang/pr/legacy-install-no-exes

Don't solve for executables in legacy install codepath.

cabal-install/Distribution/Client/Install.hs

@@ -107,6 +107,7 @@ import Distribution.Client.JobControl
 
 import qualified Distribution.Solver.Types.ComponentDeps as CD
 import           Distribution.Solver.Types.ConstraintSource
+import           Distribution.Solver.Types.Settings
 import           Distribution.Solver.Types.LabeledPackageConstraint
 import           Distribution.Solver.Types.OptionalStanza
 import qualified Distribution.Solver.Types.PackageIndex as SourcePackageIndex
@@ -423,6 +424,10 @@ planPackages comp platform mSandboxPkgInfo solver
 
       . (if reinstall then reinstallTargets else id)
 
+        -- Don't solve for executables, the legacy install codepath
+        -- doesn't understand how to install them
+      . setSolveExecutables (SolveExecutables False)
+
       $ standardInstallPolicy
         installedPkgIndex sourcePkgDb pkgSpecifiers