ENH: improve cmake/ParaView config handling

Infrastructure for handling user-specfied installation paths for cmake-based code

• the wmake/scripts/wmake.cmake-args handler traps -prefix=??? options and exports them as CMAKE_INSTALL_PATH, which can be used by cmake.
• the make sentinel file now tracks a variety of parameters (previously just ParaView_DIR and VTK_DIR)
• support plugin builds without predefining a PV_PLUG_PATH environment. Try harder to resolve the paraview-api value if the first attempt fails.
• Unbundle the cmake configuration from the paraview configuration file. Use the have_cmake wmake/script to access during build only (avoids affecting the PATH).