qt4: fix parallel building of moc_qtgradientviewdialog.o

Previously the last of these commands:

```
make -j sub-tools-bootstrap
make -j sub-uic sub-moc sub-rcc
make -C tools/designer/src/lib .obj/release-shared/moc_qtgradientviewdialog.o
```

failed with this error:

```
In file included from .uic/release-shared/ui_qtgradientviewdialog.h:63:0,
                 from .moc/release-shared/../../../../../shared/qtgradienteditor/qtgradientviewdialog.h:47,
                 from .moc/release-shared/moc_qtgradientviewdialog.cpp:9:
../../../shared/qtgradienteditor/qtgradientview.h:47:31: fatal error: ui_qtgradientview.h: No such file or directory
```

because uic did not ensure that the build of moc_qtgradientviewdialog.o happens
after the build of moc_qtgradientview.o.

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

@@ -126,6 +126,11 @@ stdenv.mkDerivation rec {
     sed -i 's/QMAKE_CXX = g++/QMAKE_CXX = clang++/' mkspecs/common/g++-base.conf
   '';
 
+  postConfigure = ''
+    echo "applying patch ${./parallel-build.patch}"
+    patch -p1 < ${./parallel-build.patch}
+  '';
+
   prefixKey = "-prefix ";
 
   configureFlags =

pkgs/development/libraries/qt-4.x/4.8/parallel-build.patch

@@ -0,0 +1,6 @@
+--- a/tools/designer/src/lib/Makefile
++++ b/tools/designer/src/lib/Makefile
+@@ -7167,2 +7167,3 @@ compiler_moc_header_clean:
+                .uic/release-shared/ui_qtgradientviewdialog.h \
++               .uic/release-shared/ui_qtgradientview.h \
+                ../../../shared/qtgradienteditor/qtgradientviewdialog.h