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-fcgi.hs

@@ -0,0 +1,16 @@
+{-# LANGUAGE CPP #-}
+import Control.Concurrent(forkIO)
+import Network.FastCGI(runFastCGI,runFastCGIConcurrent')
+
+import PGFService(cgiMain,newPGFCache,stderrToFile,logFile)
+
+main = do stderrToFile logFile
+          fcgiMain =<< newPGFCache
+
+
+fcgiMain cache =
+#ifndef mingw32_HOST_OS
+          runFastCGIConcurrent' forkIO 100 (cgiMain cache)
+#else
+          runFastCGI (cgiMain cache)
+#endif