pegtl: support finding an external pegtl library

CMake/vtkInstallCMakePackage.cmake

@@ -76,6 +76,7 @@ set(vtk_cmake_module_files
   FindOpenSlide.cmake
   FindOpenVR.cmake
   FindOSMesa.cmake
+  FindPEGTL.cmake
   FindTBB.cmake
   FindTHEORA.cmake
   Findutf8cpp.cmake

Documentation/release/dev/pegtl.md

+# pegtl
+
+VTK can now be built against an external PEGTL library.

ThirdParty/pegtl/CMakeLists.txt

-vtk_module_third_party_internal(
-  HEADER_ONLY
-  LICENSE_FILES "vtkpegtl/LICENSE"
-  VERSION       "2.8.1")
+vtk_module_third_party(
+  INTERNAL
+    HEADER_ONLY
+    LICENSE_FILES "vtkpegtl/LICENSE"
+    VERSION       "2.8.1"
+  EXTERNAL
+    PACKAGE PEGTL
+    VERSION 2.0.0
+    TARGETS PEGTL::PEGTL
+    STANDARD_INCLUDE_DIRS)
 
 configure_file(
   "${CMAKE_CURRENT_SOURCE_DIR}/vtk_pegtl.h.in"