Ensure our documentation is also found when gnucash is run in KDE

Turns out KDE is not adhering to the xdg spec after all.
It has support for the old ghelp mechanism, but when using
the help: prefix, it falls back to an internal protocol.
In that case the requested document is searched for in
<prefix>/share/doc/HTML/<lang>.
Extra caveat - the English documentation is searched for
in a directory 'en' instead of 'C'.
This is not a problem in yelp, only in khelpcenter. Unfortunately
khelpcenter is the default application to process help: prefixes
in a Kde plasma desktop, hence the workaround.

cmake/AddXdgHelpTarget.cmake

@@ -10,7 +10,16 @@
 
 function (add_xdghelp_target docname lang entities figures)
 
-    set(BUILD_DIR "${DATADIR_BUILD}/help/${lang}/${docname}")
+    set(BUILD_DIR_BASE "${DATADIR_BUILD}/help/${lang}")
+    set(BUILD_DIR "${BUILD_DIR_BASE}/${docname}")
+
+    # Define location where KDE's help system looks for <doc>
+    # when invoked with help:<doc>
+    set(kde_lang ${lang})
+    if (lang STREQUAL "C")
+        set(kde_lang "en")
+    endif()
+    set(BUILD_DIR_KDE_BASE "${DATADIR_BUILD}/doc/HTML")
 
     set(source_files "")
     foreach(xml_file ${entities} index.docbook)
@@ -33,6 +42,10 @@ function (add_xdghelp_target docname lang entities figures)
       OUTPUT "${CMAKE_CURRENT_BINARY_DIR}/xdghelptrigger"
       COMMAND ${CMAKE_COMMAND} -E make_directory "${BUILD_DIR}"
       COMMAND ${CMAKE_COMMAND} -E make_directory "${BUILD_DIR}/figures"
+
+      # Add links to make our documentation visible for KDE's help system
+      COMMAND ${CMAKE_COMMAND} -E make_directory "${BUILD_DIR_KDE_BASE}"
+      COMMAND ${CMAKE_COMMAND} -E create_symlink "${BUILD_DIR_BASE}" "${BUILD_DIR_KDE_BASE}/${kde_lang}"
       COMMAND touch "${CMAKE_CURRENT_BINARY_DIR}/xdghelptrigger")
 
 
@@ -66,10 +79,19 @@ function (add_xdghelp_target docname lang entities figures)
 
     add_dependencies(${docname}-xdghelp "${lang}-${docname}-xdghelp")
 
+    set(doc_install_dir_base "${CMAKE_INSTALL_DATADIR}/help/${lang}")
+    set(doc_install_dir "${doc_install_dir_base}/${docname}")
     install(FILES ${source_files}
-        DESTINATION "${CMAKE_INSTALL_DATADIR}/help/${lang}/${docname}"
+        DESTINATION "${doc_install_dir}"
         COMPONENT "xdghelp")
     install(FILES ${figures}
-        DESTINATION "${CMAKE_INSTALL_DATADIR}/help/${lang}/${docname}/figures"
+        DESTINATION "${doc_install_dir}/figures"
+        COMPONENT "xdghelp")
+
+    # Add links to make our documentation visible for KDE's help system
+    set(doc_install_dir_kde_base "${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_DATADIR}/doc/HTML")
+    install(CODE "execute_process (COMMAND ${CMAKE_COMMAND} -E make_directory \"${doc_install_dir_kde_base}\")"
+        COMPONENT "xdghelp")
+    install(CODE "execute_process (COMMAND ${CMAKE_COMMAND} -E create_symlink \"../../../${doc_install_dir_base}\" \"${doc_install_dir_kde_base}/${kde_lang}\")"
         COMPONENT "xdghelp")
 endfunction()