Setup.hs: don't ask darcs for the version history if _darcs is not present

This is to avoid a (harmless) error message from darcs when compiling sources
obtained e.g. from github.

1 files changed, 2 insertions, 1 deletions

diff --git a/Setup.hs b/Setup.hs
index 066b86319..8a426d1dd 100644
--- a/Setup.hs
+++ b/Setup.hs
@@ -383,7 +383,8 @@ extractDarcsVersion distFlag =
     modname = "DarcsVersion_gf"
 
     askDarcs =
-      do tags <- lines `fmap` readProcess "darcs" ["show","tags"] ""
+      do flip unless (fail "no _darcs") =<< doesDirectoryExist "_darcs"
+         tags <- lines `fmap` readProcess "darcs" ["show","tags"] ""
          let from = case tags of
                       [] -> []
                       tag:_ -> ["--from-tag="++tag]