Build: Simplify documentation target creation

We don't have to rely on qdoc being located alongside qmake anymore,
because nowadays Qt defines a Qt::doc target with IMPORTED_LOCATION set,
so we can use that directly.

Change-Id: I450578e711e0835e00320251dcadc78c1f02ab7d
Reviewed-by: Cristian Adam <cristian.adam@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>

cmake/QtCreatorDocumentation.cmake

@@ -43,23 +43,6 @@ function(qt5_query_qmake)
   endforeach()
 endfunction()
 
-# Find programs:
-function(_doc_find_program result_var)
-  if (NOT TARGET Qt::qmake)
-    message(FATAL_ERROR "QDoc is only available in Qt5 projects")
-  endif()
-
-  get_target_property(_qmake_binary Qt::qmake IMPORTED_LOCATION)
-  get_filename_component(_qmake_dir "${_qmake_binary}" DIRECTORY)
-  find_program("_prg_${result_var}" ${ARGN} HINTS "${_qmake_dir}")
-  if ("_prg_${result_var}" STREQUAL "_prg_${result_var}-NOTFOUND")
-    set("_prg_${result_var}" "${result_var}-NOTFOUND")
-    message(WARNING "Could not find binary for ${result_var}")
-  endif()
-
-  set(${result_var} "${_prg_${result_var}}" PARENT_SCOPE)
-endfunction()
-
 function(_setup_doc_targets)
   # Set up important targets:
   if (NOT TARGET html_docs)
@@ -78,6 +61,11 @@ function(_setup_qdoc_targets _qdocconf_file _retval)
   cmake_parse_arguments(_arg "" "HTML_DIR;INSTALL_DIR;POSTFIX"
     "INDEXES;INCLUDE_DIRECTORIES;FRAMEWORK_PATHS;ENVIRONMENT_EXPORTS" ${ARGN})
 
+  if (NOT TARGET Qt::qdoc)
+    message(WARNING "qdoc missing: No documentation targets were generated. Add find_package(Qt5 COMPONENTS Help) to CMake to enable.")
+    return()
+  endif()
+
   foreach(_index ${_arg_INDEXES})
     list(APPEND _qdoc_index_args "-indexdir;${_index}")
   endforeach()
@@ -90,9 +78,9 @@ function(_setup_qdoc_targets _qdocconf_file _retval)
     list(APPEND _env "${_export}=${${_export}}")
   endforeach()
 
-  set(_full_qdoc_command "${_qdoc}")
+  get_target_property(_full_qdoc_command Qt::qdoc IMPORTED_LOCATION)
   if (_env)
-    set(_full_qdoc_command "${CMAKE_COMMAND}" "-E" "env" ${_env} "${_qdoc}")
+    set(_full_qdoc_command "${CMAKE_COMMAND}" "-E" "env" ${_env} "${_full_qdoc_command}")
   endif()
 
   if (_arg_HTML_DIR STREQUAL "")
@@ -182,12 +170,6 @@ endfunction()
 function(qdoc_build_qdocconf_file _qdocconf_file)
   _setup_doc_targets()
 
-  _doc_find_program(_qdoc NAMES qdoc qdoc-qt5)
-  if (_qdoc STREQUAL "_prg__qdoc-NOTFOUND")
-     message(WARNING "No qdoc binary found: No documentation targets were generated")
-     return()
-  endif()
-
   cmake_parse_arguments(_arg "QCH" "HTML_DIR;QCH_DIR;INSTALL_DIR;POSTFIX"
     "INDEXES;INCLUDE_DIRECTORIES;FRAMEWORK_PATHS;ENVIRONMENT_EXPORTS" ${ARGN})
   if (_arg_UNPARSED_ARGUMENTS)
@@ -206,7 +188,7 @@ function(qdoc_build_qdocconf_file _qdocconf_file)
     FRAMEWORK_PATHS ${_arg_FRAMEWORK_PATHS}
   )
 
-  if (_arg_QCH)
+  if (_arg_QCH AND _html_outputdir)
     _setup_qhelpgenerator_targets("${_qdocconf_file}" "${_html_outputdir}"
       QCH_DIR "${_arg_QCH_DIR}" INSTALL_DIR "${_arg_INSTALL_DIR}")
   endif()