Allow use of external PEGTL

Not yet fully tested. Currently results in:

-- Found PEGTL: /usr/include (Required is at least version "2.4.0") 
CMake Warning at CMake/vtkModule.cmake:4750 (message):
  A third party dependency for VTK::pegtl was found externally using paths
  rather than targets; it is recommended to use imported targets rather than
  find_library and such.
Call Stack (most recent call first):
  CMake/vtkModule.cmake:4522 (vtk_module_third_party_external)
  ThirdParty/pegtl/CMakeLists.txt:1 (vtk_module_third_party)

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Backport: release