QT5.6: Fixes compile error with wrong LD env variable

QT uses -Wl linker flags and these flags only work with gcc/g++
and not with LD directly.

pkgs/development/libraries/qt-5/5.6/qtbase/default.nix

@@ -103,6 +103,8 @@ stdenv.mkDerivation {
   preConfigure = ''
     export LD_LIBRARY_PATH="$PWD/lib:$PWD/plugins/platforms:$LD_LIBRARY_PATH"
     export MAKEFLAGS=-j$NIX_BUILD_CORES
+    # We need to set LD to CXX or otherwise we get nasty compile errors
+    export LD=$CXX
 
     configureFlags+="\
         -plugindir $out/$qtPluginPrefix \