Split pgf-server into pgf-fcgi and pgf-http.

The dependency on the fastcgi package made pgf-server difficult to compile, so
it is now split into

  - pgf-fgci (main module in pgf-fcgi.hs), which is built only if fastcgi is
    already installed or if you turn on the fastcgi flag (e.g. by doing
    'cabal install -f fastcgi').

  - pgf-http (main module in pgf-http.hs) which is always built (and hopefully
    has no problematic dependencies.) 

The modules FastCGIUtils and PGFService no longer depend on fastcgi.

src/server/pgf-http.hs

@@ -0,0 +1,38 @@
+
+import Network.CGI(requestMethod,getVarWithDefault,logCGI,handleErrors,liftIO)
+import System.Environment(getArgs)
+import System.Directory(getDirectoryContents)
+import System.FilePath(takeExtension,takeFileName,takeDirectory)
+       
+import RunHTTP(runHTTP)
+import ServeStaticFile(serveStaticFile)
+import PGFService(cgiMain',getPath,stderrToFile,logFile,newPGFCache)
+import FastCGIUtils(outputJSONP,handleCGIErrors)
+
+main :: IO ()
+main = do stderrToFile logFile
+          cache <- newPGFCache
+          args <- getArgs
+          port <- case args of
+                    []     -> return 41296
+                    [port] -> readIO port
+          httpMain cache port
+
+httpMain cache port = runHTTP port (do log ; serve =<< getPath)
+  where
+    log = do method <- requestMethod
+             uri    <- getVarWithDefault "REQUEST_URI" "-"
+             logCGI $ method++" "++uri
+
+    serve path =
+        handleErrors . handleCGIErrors $
+        if takeExtension path==".pgf"
+        then cgiMain' cache path
+        else if takeFileName path=="grammars.cgi"
+             then grammarList (takeDirectory path)
+             else serveStaticFile path
+
+    grammarList dir =
+        do paths <- liftIO $ getDirectoryContents dir
+           let pgfs = [path|path<-paths, takeExtension path==".pgf"]
+           outputJSONP pgfs