io & 2d CMake configs fail to ignore VTK even when WITH_VTK set to OFF

Environment

• Operating System and version: Ubuntu 16.04
• Compiler: clang8
• PCL Version: 1.8.1

Context

PCL built with WITH_VTK=OFF
OS has VTK libraries installed, but with no intention of using them here.
Linking against PCL_IO with ${PCL_IO_LIBRARIES} after a find_package(PCL NO_MODULE REQUIRED COMPONENTS io)

Expected Behavior

PCL compiled with WITH_VTK=OFF should not attempt to link against VTK, nor cause dependants to try to link against VTK

Current Behavior

When compiling a system against a custom build of PCL with WITH_VTK=OFF, linking to PCL_IO using

target_link_libraries(mytarget ${PCL_IO_LIBRARIES})
Causes the mytarget to link against VTK despite my intentions to prevent this.

Possible Solution

I'm not a CMake expert so I hesitate to suggest a solution or to submit a pull request, but If my diagnosis is correct, then:

PCLConfig.cmake.in uses @PCLCONFIG_EXTERNAL_DEPENDENCIES@ To dump in

set(pcl_io_opt_dep openni openni2 ensenso davidSDK dssdk pcap png vtk libusb-1.0 )
set(pcl_2d_opt_dep vtk )

into PCLConfig.cmake causing anyone linking against anything dependant on pcl_io or pcl_2d to try to link against VTK.

This appears to happe because of io/CMakeLists.txt line 10 where we see:
PCL_SUBSYS_DEPEND(build "${SUBSYS_NAME}" DEPS ${SUBSYS_DEPS} OPT_DEPS openni openni2 ensenso davidSDK dssdk pcap png vtk libusb-1.0)

And 2d/CMakeLists.txt line '7' where we see:
PCL_SUBSYS_DEPEND(build "${SUBSYS_NAME}" DEPS ${SUBSYS_DEPS} OPT_DEPS vtk)

regardless of whether WITH_VTK is OFF or ON

Adding a check for WITH_VTK around that PCL_SUBSYS_DEPEND line might do the trick.

Read io/CMakeLists.txt, 2d/CMakeLists.txt, and PCLConfig.cmake.in, then inspect how PCL_SUBSYS_DEPEND records optional dependencies when WITH_VTK is OFF. Configure PCL with WITH_VTK=OFF and link a dependent target using PCL_IO_LIBRARIES. Done means that dependent target no longer attempts to link against VTK.