Make PGFService.hs compile again

+ PGFService.hs: add Nothing arguments in calls to PGF.parse_ and
  PGF.getPArseOutput
+ gf-server.cabal: mark content-server as not buildable, since required source
  files are missing.

src/server/gf-server.cabal

@@ -37,5 +37,6 @@ executable pgf-server
     ghc-options: -optl-mwindows
 
 executable content-server
+  buildable: False
   build-depends: base >=4.2 && <5
   main-is: ContentService.hs