Parallel compilation: "gf -make -j" and "gf -make -j=n" now work as expected

* "gf -make -j=n" uses n parallel threads.
  * "gf -make -j" adapts to the number of processors in the system.

This mimics how "cabal build -j" and "ghc --make -j" works.

Support for this is implemented in the new module GF.System.Concurrency and
it depends on the function Control.Concurrent.setNumCapabilities, which is
only available in GHC>=7.6 (base>=4.6). GF can still be compiled with
GHC<7.6, but then you have to use +RTS -N -RTS to take advantage of
multicore processors.

To detect the number of processors in the system, the code depends on a
foreign import of a C function in the GHC run-time system.

src/compiler/GF/System/Concurrency.hs

@@ -0,0 +1,33 @@
+{-# LANGUAGE CPP,ForeignFunctionInterface #-}
+-- | A variant of 'Control.Concurrent.setNumCapabilities' that automatically
+-- detects the number of processors in the system, and is available
+-- even when compiling with GHC<7.6.
+module GF.System.Concurrency(
+  -- * Controlling parallelism
+  setNumCapabilities,getNumberOfProcessors) where
+import qualified Control.Concurrent as C
+import Foreign.C.Types(CInt(..))
+
+
+
+
+-- | Set parallelism to a given number, or use the number of processors.
+-- Returns 'False' if compiled with GHC<7.6 and the desired number of threads
+-- hasn't already been set with @+RTS -N/n/ -RTS@.
+setNumCapabilities opt_n =
+  do n <- maybe getNumberOfProcessors return opt_n
+#if MIN_VERSION_base(4,6,0)
+     C.setNumCapabilities n
+     return True
+#else
+     n_now <- C.getNumCapabilities
+     print (n,n_now)
+     return (n==n_now)
+#endif
+
+-- | Returns the number of processors in the system.
+getNumberOfProcessors = fmap fromEnum c_getNumberOfProcessors
+
+-- | According to comments in cabal-install cbits/getnumprocessors.c
+-- this function is part of the RTS of GHC>=6.12.
+foreign import ccall "getNumberOfProcessors" c_getNumberOfProcessors :: IO CInt