diff --git a/Setup.hs b/Setup.hs
--- a/Setup.hs
+++ b/Setup.hs
@@ -5,6 +5,8 @@
 import Distribution.Simple.Utils
 import Distribution.PackageDescription
 import System.Environment
+import Data.List (partition,stripPrefix,foldl')
+import Data.Maybe (mapMaybe)
 
 getBuildInfo :: LocalBuildInfo -> BuildInfo
 getBuildInfo l = case library (localPkgDescr l) of
@@ -19,43 +21,89 @@
         li' = li { libBuildInfo = b }
         lpd' = lpd { library = Just li' }
 
-main :: IO ()
-main = defaultMainWithHooks myhooks
+validflags :: [String]
+validflags = ["ruby18", "ruby19", "ruby20", "ruby21"]
+
+can'tFindRuby :: String
+can'tFindRuby = unlines $ [ "Could not find the ruby library. Ensure that it is present on your system (on Debian/Ubuntu, make sure you installed the ruby1.8-dev package)."
+                          , "If you know it to be installed, please install hruby in the following way:"
+                          , ""
+                          , "$ cabal install hruby -p --configure-option='--rubylib=ruby --rubyinc=/usr/include/ruby-2.1.0/x86_64-linux --rubyversion=21'"
+                          , ""
+                          , " --rubylib : Should be the name of the library passed to the linker (ruby for libruby.so)."
+                          , " --rubyinc : There can be several instances of this flag. Should be the path of the various ruby header files."
+                          , " --rubyversion : Mandatory for ruby 2.0 and 2.1, should have the values 20 or 21."
+                          ]
+
+type InstallInfo = (String, [String], [String])
+
+inc18,inc19,inc20,inc21 :: [String]
+inc21 = ["/usr/include/ruby-2.1.0", "/usr/include/x86_64-linux-gnu/ruby-2.1.0", "/usr/include/ruby-2.1.0/x86_64-linux"]
+inc20 = ["/usr/include/ruby-2.0.0", "/usr/include/x86_64-linux-gnu/ruby-2.0.0", "/usr/include/ruby-2.0.0/x86_64-linux"]
+inc19 = ["/usr/lib/ruby/1.9/x86_64-linux"]
+inc18 = ["/usr/lib/ruby/1.8/x86_64-linux"]
+
+r18,r19,r20,r21 :: InstallInfo
+r21 = ("ruby2.1", inc21, ["-DRUBY2","-DRUBY21"])
+r20 = ("ruby2.0", inc20, ["-DRUBY2"])
+r19 = ("ruby1.9", inc19, [])
+r18 = ("ruby1.8", inc18, [])
+
+defaultLib :: InstallInfo -> InstallInfo
+defaultLib (_,i,c) = ("ruby",i,c)
+
+myConfHook :: LocalBuildInfo -> IO LocalBuildInfo
+myConfHook h = do
+    -- hunt for libraries
+    let rubies = ["ruby2.1", "ruby2.0", "ruby1.9", "ruby1.8", "ruby"]
+        pathes = ["/usr/lib", "/usr/local/lib"]
+    f <- findFirstFile (\(p,r) -> p ++ "/lib" ++ r ++ ".so") [(p,r) | p <- pathes, r <- rubies]
+    let rubystring = maybe "" snd f
+        lg s = notice normal ("Auto detected " ++ s)
+    case fmap snd f of
+        Just "ruby2.1" -> hookWith r21 h
+        Just "ruby2.0" -> hookWith r20 h
+        Just "ruby1.9" -> hookWith r19 h
+        Just "ruby1.8" -> hookWith r18 h
+        Just "ruby" -> do
+            is21 <- findFirstFile id inc21
+            is20 <- findFirstFile id inc20
+            is19 <- findFirstFile id inc19
+            is18 <- findFirstFile id inc18
+            case filter ((/= Nothing) . fst) [(is21,r21), (is20,r20), (is19,r19), (is18,r18)] of
+                ((_,(n,inc,flgs)):_) -> hookWith ("ruby",inc,flgs) h
+                _ -> warn normal can'tFindRuby >> return h
+        Just x -> die $ "Did find this (this should not happen): " ++ x
+        _ -> warn normal can'tFindRuby >> return h
+
+hookWith (e,i,c) h =
+    let buildinfos = getBuildInfo h
+        bi = buildinfos { extraLibs = e : extraLibs buildinfos
+                        , includeDirs = includeDirs buildinfos ++ i
+                        , ccOptions = ccOptions buildinfos ++ c
+                        }
+    in putStrLn ("Detected parameters: " ++ show (e,i,c)) >> return (setBuildInfo h bi)
+
+parseFlags :: [String] -> LocalBuildInfo -> IO LocalBuildInfo
+parseFlags flags h = return $ setBuildInfo h $ foldl' parseFlags' bbi flags
     where
-        myhooks    = simpleUserHooks { confHook = (\a b -> configure a b >>= myConfHook) }
-        myConfHook h = do
-            let buildinfos = getBuildInfo h
-            -- hunt for libraries
-            let rubies = ["ruby2.1", "ruby2.0", "ruby1.9", "ruby1.8", "ruby"]
-                pathes = ["/usr/lib", "/usr/local/lib"]
-                inc21 = ["/usr/include/ruby-2.1.0", "/usr/include/x86_64-linux-gnu/ruby-2.1.0", "/usr/include/ruby-2.1.0/x86_64-linux"]
-                inc20 = ["/usr/include/ruby-2.0.0", "/usr/include/x86_64-linux-gnu/ruby-2.0.0", "/usr/include/ruby-2.0.0/x86_64-linux"]
-                inc19 = ["/usr/lib/ruby/1.9/x86_64-linux"]
-                inc18 = ["/usr/lib/ruby/1.8/x86_64-linux"]
-                r21 = ("ruby2.1", inc21, ["-DRUBY2","-DRUBY21"])
-                r20 = ("ruby2.0", inc20, ["-DRUBY2"])
-                r19 = ("ruby1.9", inc19, [])
-                r18 = ("ruby1.8", inc18, [])
-            f <- findFirstFile (\(p,r) -> p ++ "/lib" ++ r ++ ".so") [(p,r) | p <- pathes, r <- rubies]
-            let rubystring = maybe "" snd f
-                lg s = notice normal ("Auto detected " ++ s)
-            (e,i,c) <- case fmap snd f of
-                           Just "ruby2.1" -> lg rubystring >> return r21
-                           Just "ruby2.0" -> lg rubystring >> return r20
-                           Just "ruby1.9" -> lg rubystring >> return r19
-                           Just "ruby1.8" -> lg rubystring >> return r18
-                           Just "ruby" -> do
-                                is21 <- findFirstFile id inc21
-                                is20 <- findFirstFile id inc20
-                                is19 <- findFirstFile id inc19
-                                is18 <- findFirstFile id inc18
-                                case filter ((/= Nothing) . fst) [(is21,r21), (is20,r20), (is19,r19), (is18,r18)] of
-                                    ((_,(n,inc,flgs)):_) -> lg n >> return ("ruby",inc,flgs)
-                                    _ -> die "Could not find the ruby library"
-                           Just x -> die $ "Did find this (this should not happen): " ++ x
-                           _ -> die "Could not find the ruby library"
-            let bi = buildinfos { extraLibs = e : extraLibs buildinfos
-                                , includeDirs = includeDirs buildinfos ++ i
-                                , ccOptions = ccOptions buildinfos ++ c
-                                }
-            return (setBuildInfo h bi)
+        bbi = getBuildInfo h
+        parseFlags' bi fl = case (stripPrefix "--rubylib=" fl, stripPrefix "--rubyinc=" fl, stripPrefix "--rubyversion=" fl) of
+                                (Just l, _, _)    -> bi { extraLibs   = l : extraLibs bi }
+                                (_, Just l, _)    -> bi { includeDirs = l : extraLibs bi }
+                                (_, _, Just "20") -> bi { ccOptions   = "-DRUBY2" : ccOptions bi }
+                                (_, _, Just "21") -> bi { ccOptions   = "-DRUBY2" : "-DRUBY21" : ccOptions bi }
+                                _                 -> bi
+            where
+                rubylib = stripPrefix "--rubylib=" fl
+                rubyinc = stripPrefix "--rubyinc=" fl
+
+main :: IO ()
+main = do
+    args <- getArgs
+    let flags = mapMaybe (stripPrefix "--configure-option=") args
+    let hook = if null flags
+                   then myConfHook
+                   else parseFlags (concatMap words flags)
+    defaultMainWithHooks $ simpleUserHooks { confHook = (\a b -> configure a b >>= hook) }
+
diff --git a/hruby.cabal b/hruby.cabal
--- a/hruby.cabal
+++ b/hruby.cabal
@@ -2,7 +2,7 @@
 --  see http://haskell.org/cabal/users-guide/
 
 name:                hruby
-version:             0.2.3
+version:             0.2.4
 synopsis:            Embed Ruby in your Haskell program.
 description:         Warning: this is completely experimental. Everything you need should be in "Foreign.Ruby".
 license:             BSD3