cmake: Import VTKCompileTools if CROSSCOMPILING_EMULATOR support is incomplete

Suggested-by: Ben Boeckel <ben.boeckel@kitware.com>

CMakeLists.txt

@@ -248,6 +248,13 @@ if(NOT VTK_BUILD_PYTHON_MODULE_DIR)
        PATH "Directory where python modules will be put inside the build tree")
   mark_as_advanced(VTK_BUILD_PYTHON_MODULE_DIR)
 endif()
+# Full functional CMAKE_CROSSCOMPILING_EMULATOR support for custom_command and
+# custom_target is available in CMake 3.8.0
+# It was first added in CMake 3.6.0 and later fixed in CMake 3.8.0 (commit e7480d67, CMake issue #16288)
+set(_crosscompiling_emulator_support_custom_target 1)
+if(CMAKE_VERSION VERSION_LESS 3.8.0)
+  set(_crosscompiling_emulator_support_custom_target 0)
+endif()
 # Maintain backward compatibility with user setting COMPILE_TOOLS_IMPORTED
 if(DEFINED COMPILE_TOOLS_IMPORTED AND NOT DEFINED VTK_COMPILE_TOOLS_IMPORTED)
   set(VTK_COMPILE_TOOLS_IMPORTED ${COMPILE_TOOLS_IMPORTED})
@@ -256,7 +263,11 @@ endif()
 if(NOT DEFINED VTK_COMPILE_TOOLS_IMPORTED)
   set(VTK_COMPILE_TOOLS_IMPORTED FALSE)
 endif()
-if(CMAKE_CROSSCOMPILING AND NOT VTK_COMPILE_TOOLS_IMPORTED AND NOT DEFINED CMAKE_CROSSCOMPILING_EMULATOR)
+if(CMAKE_CROSSCOMPILING
+    AND NOT VTK_COMPILE_TOOLS_IMPORTED
+    AND NOT DEFINED CMAKE_CROSSCOMPILING_EMULATOR
+    AND NOT _crosscompiling_emulator_support_custom_target
+  )
   # if CMAKE_CROSSCOMPILING is true and crosscompiling emulator is not available, we need
   # to import build-tools targets.
   find_package(VTKCompileTools REQUIRED)