Remove CMake policy CMP0054 setting.

[SergioRAgostinho]

I compiled, ran tests and linked against a downstream target. It seems our code was already compliant and it didn't cause any issues.

At some point the PCLConfig was reporting "duplicate targets" again. After deleting and rebuilding I could no longer replicate this situation. I would still suggest other @PointCloudLibrary/maintainers to actually give it a try on their local platforms before merging.

[taketwo]

I searched for occurrences of if(" in our CMake scripts and whatever I found does not seem to be using the old "double-expansion" behavior. I also built/installed PCL and tested with one dependent project, works fine.

[SunBlack]

I don't know if this warning is new since this PR, but I'm getting always:

CMake Warning (dev) at /usr/lib/cmake/vtk-6.3/VTKTargets.cmake:28 (if):
  Policy CMP0054 is not set: Only interpret if() arguments as variables or
  keywords when unquoted.  Run "cmake --help-policy CMP0054" for policy
  details.  Use the cmake_policy command to set the policy and suppress this
  warning.

  Quoted variables like "" will no longer be dereferenced when the policy is
  set to NEW.  Since the policy is not set the OLD behavior will be used.
Call Stack (most recent call first):
  /usr/lib/cmake/vtk-6.3/VTKConfig.cmake:60 (include)
  CMakeLists.txt:368 (find_package)
This warning is for project developers.  Use -Wno-dev to suppress it.

Don't know why we don't see it on Azure build machines. I have a VM with Ubuntu 18.04 and all dependencies installed via apt-get:

sudo apt-get install build-essential clang ccache cmake-qt-gui freeglut3-dev libglew-dev qt5-default libvtk6-qt-dev libusb-1.0-0-dev libopenni-dev libopenni2-dev libboost-all-dev libeigen3-dev libflann-dev googletest