WebSetup.hs: always enable parallel compilation of the RGL and the example grammars

This should work nicely now that GF itself can detect the number of processors
in the system.

Previously there was some code in WebSetup.hs that relied on Cabal to do this,
but it was commented out because it requires Cabal>=1.20, which is fairly new
and it would complicate the installation instructions if it was required to
compile GF...

WebSetup.hs

@@ -102,9 +102,10 @@ execute command args =
 -- to avoid casing problems for developers using Cabal<1.20
 numJobs flags =
     if null n
-    then ["-j=1"]
+    then ["-j","+RTS","-A20M","-N","-RTS"]
     else ["-j="++n,"+RTS","-A20M","-N"++n,"-RTS"]
   where
+    -- buildNumJobs is only available in Cabal>=1.20
     n = {-case buildNumJobs flags of
           Flag mn | mn/=Just 1-> maybe "" show mn
           _ ->-} ""