COMP: Fix windows compilation with Slicer_BUILD_PARAMETERSERIALIZER_SUPPORT

The JsonCpp_LIBRARY needs to be passed to SlicerExecutionModel as a
CMAKE_ARGS because it contains the result of the ${CMAKE_CFG_INTDIR}
variable that needs to be evaluated.
To clarify and for future improvements, the EXTERNAL_PROJECT_OPTIONAL_ARGS
variable was renamed EXTERNAL_PROJECT_OPTIONAL_CMAKE_CACHE_ARGS and the
EXTERNAL_PROJECT_OPTIONAL_CMAKE_ARGS was created.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25351 3bd1e089-480b-0410-8dfb-8563597acbee