COMP: Fix Slicer extension build by not forcing CMAKE_CXX_STANDARD value

[jcfr]

This commit fixes the following error introduced by Slicer/Slicer@dedb6c04d5 (COMP: Require at least C++17 (PR-6237))

  -- Checking if building as a Slicer extension - yes (DCMQI_BUILD_SLICER_EXTENSION is ON)
  CMake Error at /work/Preview/Slicer-0-build/Slicer-build/SlicerConfig.cmake:969 (message):
    Variable CMAKE_CXX_STANDARD defined prior calling 'find_package(Slicer)'
    does NOT match value used to configure Slicer.  It probably means that a
    different CMAKE_CXX_STANDARD has been used to configure this project and
    Slicer.

    CMAKE_CXX_STANDARD=14

    Slicer_CMAKE_CXX_STANDARD=17
  Call Stack (most recent call first):
    /work/Preview/Slicer-0-build/Slicer-build/SlicerConfig.cmake:983 (slicer_config_set_compiler_ep)
    CMakeLists.txt:233 (find_package)

cc: @sjh26 @jamesobutler

[jcfr]

Thanks @fedorov :pray:

[fedorov]

Thank YOU @jcfr!