Fix a bug where the destdir prefix got included in the prog prefix and suffix

Previously all members of InstallDirs were paths so it was safe to prefix
them all with the destdir, but now we've also got these two non-paths, so
we have to not apply the prefix to them. Perhaps the program prefix and suffix
should just be PathTemplates on their own and not part of InstallDirs.
Reported by MarcWeber while hacking on Haskell packages for Nix.

Distribution/Simple/InstallDirs.hs

@@ -313,7 +313,13 @@ absoluteInstallDirs :: PackageIdentifier -> PackageIdentifier -> CopyDest
                     -> InstallDirTemplates -> InstallDirs FilePath
 absoluteInstallDirs pkgId compilerId copydest dirs =
     (case copydest of
-       CopyTo destdir -> fmap ((destdir </>) . dropDrive)
+       CopyTo destdir -> \dirs -> (fmap ((destdir </>) . dropDrive) dirs) {
+                            -- We add the destdir to all the paths, but the
+                            -- program prefix and suffix are not paths, so we
+                            -- keep their old values:
+                            progprefix = progprefix dirs,
+                            progsuffix = progsuffix dirs
+                          }
        _              -> id)
   . appendSubdirs (</>)
   . fmap fromPathTemplate