Help: Fix QtHelp commands on Windows

Explicitly invoke python script through the interpreter since
windows does not act on hashbangs.
Use the found qcollectiongenerator executable rather than what
happens to be in PATH.

Utilities/Sphinx/CMakeLists.txt

@@ -69,6 +69,8 @@ if(SPHINX_TEXT)
   list(APPEND doc_formats text)
 endif()
 if(SPHINX_QTHELP)
+  find_package(PythonInterp REQUIRED)
+
   find_program(QCOLLECTIONGENERATOR_EXECUTABLE
     NAMES qcollectiongenerator
     DOC "qcollectiongenerator tool"
@@ -91,9 +93,12 @@ if(SPHINX_QTHELP)
 
     # Create proper identifiers. Workaround for
     # https://bitbucket.org/birkenfeld/sphinx/issue/1491/qthelp-should-generate-identifiers-for
-    COMMAND "${CMAKE_CURRENT_SOURCE_DIR}/create_identifiers.py" "${CMAKE_CURRENT_BINARY_DIR}/qthelp/"
+    COMMAND "${PYTHON_EXECUTABLE}"
+      "${CMAKE_CURRENT_SOURCE_DIR}/create_identifiers.py"
+      "${CMAKE_CURRENT_BINARY_DIR}/qthelp/"
 
-    COMMAND qcollectiongenerator ${CMAKE_CURRENT_BINARY_DIR}/qthelp/CMake.qhcp
+    COMMAND ${QCOLLECTIONGENERATOR_EXECUTABLE}
+      ${CMAKE_CURRENT_BINARY_DIR}/qthelp/CMake.qhcp
   )
 endif()