Problem: Broken documentation build

Solution: The CMAKE module PythonInterp sets the variable
PYTHONINTERP_FOUND and not PYTHON_FOUND if it finds
a python interpreter.

CMakeLists.txt

@@ -329,7 +329,7 @@ find_package (PythonInterp)
 find_package (AsciiDoc)
 
 cmake_dependent_option (WITH_DOC "Build Reference Guide documentation (requires DocBook)" ON
-                       "PYTHON_FOUND;ASCIIDOC_FOUND" OFF)
+                       "PYTHONINTERP_FOUND;ASCIIDOC_FOUND" OFF)
 
 if (MSVC)
   if (WITH_OPENPGM)