ExternalModule: Ensure module built externally use required VTK flags

CMake/vtkExternalModuleMacros.cmake

@@ -45,6 +45,17 @@ endif()
 
 include(${VTK_CMAKE_DIR}/vtkInitializeBuildType.cmake)
 
+# Use VTK's flags.
+set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${VTK_REQUIRED_C_FLAGS}")
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${VTK_REQUIRED_CXX_FLAGS}")
+set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} ${VTK_REQUIRED_EXE_LINKER_FLAGS}")
+set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} ${VTK_REQUIRED_SHARED_LINKER_FLAGS}")
+set(CMAKE_MODULE_LINKER_FLAGS "${CMAKE_MODULE_LINKER_FLAGS} ${VTK_REQUIRED_MODULE_LINKER_FLAGS}")
+option(BUILD_SHARED_LIBS "Build VTK module with shared libraries." ${VTK_BUILD_SHARED_LIBS})
+if(NOT CMAKE_POSITION_INDEPENDENT_CODE)
+  set(CMAKE_POSITION_INDEPENDENT_CODE ON)
+endif()
+
 # Add the VTK_MODULES_DIR to the CMAKE_MODULE_PATH and then use the binary
 # directory for the project to write out new ones to.
 if(VTK_MODULES_DIR)