qt48: maintenance update, try enableParallelBuilding

I successfully built it with 8 free cores;
hopefully it won't give problems.

/cc maintainers: @lovek323, @phreedom, @sander.

I wonder that the build reports no support for gstreamer and pulseaudio.
I checked it was so before this commit, but it might still be useful
to enable those.

No more 4.8.x releases are planned. End of support is scheduled to Dec'15.

pkgs/development/libraries/qt-4.x/4.8/default.nix

@@ -17,7 +17,7 @@ with stdenv.lib;
 
 let
   v_maj = "4.8";
-  v_min = "6";
+  v_min = "7";
   vers = "${v_maj}.${v_min}";
 in
 
@@ -31,7 +31,7 @@ stdenv.mkDerivation rec {
   src = fetchurl {
     url = "http://download.qt-project.org/official_releases/qt/"
       + "${v_maj}/${vers}/qt-everywhere-opensource-src-${vers}.tar.gz";
-    sha256 = "0b036iqgmbbv37dgwwfihw3mihjbnw3kb5kaisdy0qi8nn8xs54b";
+    sha256 = "183fca7n7439nlhxyg1z7aky0izgbyll3iwakw4gwivy16aj5272";
   };
 
   # The version property must be kept because it will be included into the QtSDK package name
@@ -126,9 +126,7 @@ stdenv.mkDerivation rec {
 
   nativeBuildInputs = [ perl pkgconfig which ];
 
-  # occasional build problems if one has too many cores (like on Hydra)
-  # @vcunat has been unable to find a *reliable* fix
-  enableParallelBuilding = false;
+  enableParallelBuilding = true;
 
   NIX_CFLAGS_COMPILE = optionalString stdenv.isDarwin
     "-I${glib}/include/glib-2.0 -I${glib}/lib/glib-2.0/include";