qt5: specify docdir

Due to a bug somewhere, the qt documentation is by default placed in
$out/share/doc although "qmake -query" reports $out/doc. That
discrepancy causes qtcreator withDocumentation builds to fail. This
fixes it.

pkgs/development/libraries/qt-5/5.3/default.nix

···
     export LD_LIBRARY_PATH="$PWD/qtbase/lib:$PWD/qtbase/plugins/platforms:$PWD/qttools/lib:$LD_LIBRARY_PATH"
     export MAKEFLAGS=-j$NIX_BUILD_CORES
     export configureFlags+="-plugindir $out/lib/qt5/plugins -importdir $out/lib/qt5/imports -qmldir $out/lib/qt5/qml"
+    export configureFlags+=" -docdir $out/share/doc/qt5"
   '';
 
   prefixKey = "-prefix ";

pkgs/development/libraries/qt-5/5.4/qtbase.nix

···
     sed -i 's/PATHS.*NO_DEFAULT_PATH//' "qtbase/mkspecs/features/data/cmake/Qt5BasicConfig.cmake.in"
 
     export configureFlags+="-plugindir $out/lib/qt5/plugins -importdir $out/lib/qt5/imports -qmldir $out/lib/qt5/qml"
+    export configureFlags+=" -docdir $out/share/doc/qt5"
   '';
 
   prefixKey = "-prefix ";