ENH: Do not build doc in VisualStudio when F7 is pressed

If doxygen was installed then cpp-doc was always re-built when "Build Solution" (F7) was used in Visual Studio, taking several minutes.
The change only effects the Visual Studio IDE and is independent from the inclusion of the doc target in ALL target.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24158 3bd1e089-480b-0410-8dfb-8563597acbee