vtkModule: support optional dependencies

They are treated as private dependencies to incentivize not making
optional public dependencies.

CMake/vtkModule.cmake

@@ -102,6 +102,8 @@ supported:
     using this module.
   * `PRIVATE_DEPENDS`: A list of modules which are required by this module, but
     not by those using this module.
+  * `OPTIONAL_DEPENDS`: A list of modules which are used by this module if
+    enabled; these are treated as `PRIVATE_DEPENDS` if they exist.
   * `IMPLEMENTS`: A list of modules for which this module needs to register
     with.
   * `TEST_DEPENDS`: Modules required by the test suite for this module.
@@ -128,7 +130,7 @@ macro (_vtk_module_parse_module_args name_output)
   cmake_parse_arguments("${_name_NAME}"
     "IMPLEMENTABLE;EXCLUDE_WRAP;THIRD_PARTY"
     "NAME;KIT"
-    "GROUPS;DEPENDS;PRIVATE_DEPENDS;TEST_DEPENDS;TEST_LABELS;DESCRIPTION;CONDITION;IMPLEMENTS"
+    "GROUPS;DEPENDS;PRIVATE_DEPENDS;OPTIONAL_DEPENDS;TEST_DEPENDS;TEST_LABELS;DESCRIPTION;CONDITION;IMPLEMENTS"
     ${ARGN})
 
   if (${_name_NAME}_UNPARSED_ARGUMENTS)
@@ -473,6 +475,9 @@ function (vtk_module_scan)
     set_property(GLOBAL
       PROPERTY
         "_vtk_module_${_vtk_scan_module_name}_private_depends" "${${_vtk_scan_module_name}_PRIVATE_DEPENDS}")
+    set_property(GLOBAL
+      PROPERTY
+        "_vtk_module_${_vtk_scan_module_name}_optional_depends" "${${_vtk_scan_module_name}_OPTIONAL_DEPENDS}")
     set_property(GLOBAL
       PROPERTY
         "_vtk_module_${_vtk_scan_module_name}_test_depends" "${${_vtk_scan_module_name}_TEST_DEPENDS}")
@@ -1055,15 +1060,23 @@ function (vtk_module_build)
       PROPERTY "_vtk_module_${_vtk_build_module}_depends")
     get_property("_vtk_build_${_vtk_build_module}_private_depends" GLOBAL
       PROPERTY "_vtk_module_${_vtk_build_module}_private_depends")
+    get_property("_vtk_build_${_vtk_build_module}_optional_depends" GLOBAL
+      PROPERTY "_vtk_module_${_vtk_build_module}_optional_depends")
     set("_vtk_build_${_vtk_build_module}_all_depends"
       ${_vtk_build_${_vtk_build_module}_depends}
-      ${_vtk_build_${_vtk_build_module}_private_depends})
+      ${_vtk_build_${_vtk_build_module}_private_depends}
+      ${_vtk_build_${_vtk_build_module}_optional_depends})
   endforeach ()
 
   set(_vtk_build_sorted_modules "${_vtk_build_MODULES}")
   topological_sort(_vtk_build_sorted_modules "_vtk_build_" "_all_depends")
 
   foreach (_vtk_build_module IN LISTS _vtk_build_sorted_modules)
+    list(FIND _vtk_build_MODULES "${_vtk_build_module}" _vtk_build_idx)
+    if (_vtk_build_idx EQUAL "-1")
+      continue ()
+    endif ()
+
     if (TARGET "${_vtk_build_module}")
       get_property(_vtk_build_is_imported
         TARGET    "${_vtk_build_module}"
@@ -1778,6 +1791,22 @@ function (vtk_module_add_module name)
         ${_vtk_add_module_depends}
       PRIVATE
         ${_vtk_add_module_private_depends})
+
+    get_property(_vtk_add_module_optional_depends GLOBAL
+      PROPERTY  "_vtk_module_${_vtk_build_module}_optional_depends")
+    foreach (_vtk_add_module_optional_depend IN LISTS _vtk_add_module_optional_depends)
+      if (TARGET "${_vtk_add_module_optional_depend}")
+        set(_vtk_add_module_have_optional_depend 1)
+        target_link_libraries("${_vtk_add_module_real_target}"
+          PRIVATE
+            "${_vtk_add_module_optional_depend}")
+      else ()
+        set(_vtk_add_module_have_optional_depend 0)
+      endif ()
+      target_compile_definitions("${_vtk_add_module_real_target}"
+        PRIVATE
+          "VTK_MODULE_ENABLE_${_vtk_add_module_optional_depend}=${_vtk_add_module_have_optional_depend}")
+    endforeach ()
   endif ()
   _vtk_module_standard_includes(
     ${_vtk_add_module_includes_interface}