gf.cabal: add GF builtin default -K64M. Enable parallel compile with ghc>=7.8

+ Programs compiled with ghc<7.8 have a default built-in 8M stack size limit.
  With ghc>7.0 this built-in default can be changed, so it is now 64M, allowing
  GF to work with larger grammars without manually increasing the stack size.
  (But if GF is compiled with ghc 6.12, the built-in default is still 8M.)

+ Enable ghc>=7.8 support for compiling modules in parallel, to speed up the
  compilation of GF.

gf.cabal

@@ -88,6 +88,8 @@ Library
     build-depends: binary
 
   ghc-options: -fwarn-unused-imports
+  if impl(ghc>=7.8)
+    ghc-options: -j +RTS -A20M -RTS
   ghc-prof-options: -auto-all
   extensions: 
 
@@ -157,13 +159,15 @@ Executable gf
 
   if flag(c-runtime)
     cpp-options: -DC_RUNTIME
---if flag(new-comp)
---  cpp-options: -DNEW_COMP
 
-  build-tools: happy
+  if impl(ghc>=7.0)
+    ghc-options: -rtsopts
+    if impl(ghc<7.8)
+      ghc-options: -with-rtsopts=-K64M
 
   if impl(ghc>=7.8)
     build-tools: happy>=1.19, alex>=3.1
+    ghc-options: -j +RTS -A20M -RTS
   else
     build-tools: happy, alex>=3
 
@@ -174,8 +178,6 @@ Executable gf
 
   ghc-prof-options: -auto-all
 
-  if impl(ghc>=7.0)
-    ghc-options: -rtsopts
   hs-source-dirs: src/compiler
 
   if !flag(custom-binary)