Merge pull request #3939 from ezyang/pr/lower-compat-Cabal-bound

Lower library compatibility version to 1.25, since 1.25 is dev series…

cabal-install/Distribution/Client/ProjectPlanning.hs

@@ -1112,7 +1112,7 @@ elaborateInstallPlan verbosity platform compiler compilerprogdb pkgConfigDB
             -- Only non-Custom or sufficiently recent Custom
             -- scripts can be build per-component.
             = (fromMaybe PD.Custom (PD.buildType pd) /= PD.Custom)
-                || PD.specVersion pd >= mkVersion [2,0,0]
+                || PD.specVersion pd >= mkVersion [1,25,0]
             -}
 
         elab0 = elaborateSolverToCommon mapDep spkg
@@ -2357,9 +2357,7 @@ defaultSetupDeps compiler platform pkg =
           -- breaking changes to the Cabal API that Setup.hs scripts use.
           -- So for old custom Setup scripts that do not specify explicit
           -- constraints, we constrain them to use a compatible Cabal version.
-          -- The exact version where we'll make this API break has not yet been
-          -- decided, so for the meantime we guess at 2.x.
-          cabalCompatMaxVer = mkVersion [2]
+          cabalCompatMaxVer = mkVersion [1,25]
           -- In principle we can talk to any old Cabal version, and we need to
           -- be able to do that for custom Setup scripts that require older
           -- Cabal lib versions. However in practice we have currently have