Make a documentation-only build possible

Packagers usually build Qt per repo, but the documentation must be built
from the full Qt source archive to ensure that inter-repo links are set
up correctly. When building the documentation, it's desirable to avoid
building tools and re-use to tools of an existing Qt build.

One now can do a documentation-only build by configuring Qt like this:

    cmake /path/to/Qt/source -GNinja \
          -DQT_HOST_PATH=/path/to/Qt/installation

and build the documentation with

    ninja docs

To avoid building tools, this patch removes the DEPENDS arguments from
some add_custom_target calls in QtDocsHelpers.cmake and makes another
one conditional. The removed dependencies are added at the end of
qt_internal_add_docs. Adding of tool dependencies is not done if
QT_HOST_PATH is set.

Fixes: QTBUG-88441
Change-Id: I3b7a908e22d252d2edcdc1dd522a78b8ad6c487e
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

cmake/QtDocsHelpers.cmake

@@ -31,7 +31,11 @@ function(qt_internal_add_docs)
         return()
     endif()
 
-    if(QT_SUPERBUILD)
+    set(tool_dependencies_enabled TRUE)
+    if(NOT "${QT_HOST_PATH}" STREQUAL "")
+        set(tool_dependencies_enabled FALSE)
+        set(doc_tools_dir "${QT_HOST_PATH}/${QT${PROJECT_VERSION_MAJOR}_HOST_INFO_BINDIR}")
+    elseif(QT_SUPERBUILD)
         set(doc_tools_dir "${QtBase_BINARY_DIR}/${INSTALL_BINDIR}")
     else()
         set(doc_tools_dir "${QT_BUILD_INTERNALS_RELOCATABLE_INSTALL_PREFIX}/${INSTALL_BINDIR}")
@@ -85,7 +89,6 @@ function(qt_internal_add_docs)
 
     # qtattributionsscanner
     add_custom_target(qattributionsscanner_${target}
-        DEPENDS ${qattributionsscanner_bin}
         COMMAND ${qtattributionsscanner_bin}
         ${PROJECT_SOURCE_DIR}
         --filter "QDocModule=${doc_target}"
@@ -120,7 +123,6 @@ function(qt_internal_add_docs)
     )
 
     add_custom_target(prepare_docs_${target}
-        DEPENDS ${qdoc_bin}
         COMMAND ${CMAKE_COMMAND} -E env ${qdoc_env_args}
         ${qdoc_bin}
         ${prepare_qdoc_args}
@@ -139,8 +141,12 @@ function(qt_internal_add_docs)
     )
 
     foreach(target_prefix generate_top_level_docs generate_repo_docs generate_docs)
+        set(depends_arg "")
+        if(tool_dependencies_enabled)
+            set(depends_arg DEPENDS ${qdoc_bin})
+        endif()
         add_custom_target(${target_prefix}_${target}
-            DEPENDS ${qdoc_bin}
+            ${depends_arg}
             COMMAND ${CMAKE_COMMAND} -E env ${qdoc_env_args} ${qdoc_bin} ${generate_qdocs_args})
     endforeach()
 
@@ -158,8 +164,12 @@ function(qt_internal_add_docs)
     set(qch_file_path ${qdoc_qch_output_dir}/${qch_file_name})
 
     foreach(target_prefix qch_top_level_docs qch_repo_docs qch_docs)
+        set(depends_arg "")
+        if(tool_dependencies_enabled)
+            set(depends_arg DEPENDS ${qhelpgenerator_bin})
+        endif()
         add_custom_target(${target_prefix}_${target}
-            DEPENDS ${qhelpgenerator_bin}
+            ${depends_arg}
             COMMAND ${qhelpgenerator_bin}
             "${qdoc_output_dir}/${doc_target}.qhp"
             -o "${qch_file_path}"
@@ -218,7 +228,9 @@ function(qt_internal_add_docs)
 
     # Make sure that the necessary tools are built when running,
     # for example 'cmake --build . --target generate_docs'.
+    if(tool_dependencies_enabled)
         qt_internal_add_doc_tool_dependency(qattributionsscanner_${target} qtattributionsscanner)
         qt_internal_add_doc_tool_dependency(prepare_docs_${target} qdoc)
         qt_internal_add_doc_tool_dependency(qch_docs_${target} qhelpgenerator)
+    endif()
 endfunction()