[Cmake] Target options?

[John Biddiscombe]

To improve vtk build times, I've modified my local WrapTcl to include some rudimentary precompiled header support. To make it work, I need to add an option like the following
  -H="$ENV{TEMP}/vtk/vtk.csm"
to the build (CXX flags).

If I add this command to LocalUserOptions in VTK\Common (&etc), the tcl builds like lightning, but it gets added to the standard VTK\Commmon build too, so the vtkCommon.dll builds slowly as it doesn't have the source code mods and tries to update the precompiled header symbol file on every source.

How can I add an option to a target like the tcl build, but not add it to the other targets?

thanks

JB