BUG: Mantis-856: Fix LibKML import script and enhance documentation

CMake/ImportLibKML.cmake

@@ -5,15 +5,22 @@ if(LIBKML_FOUND)
   option(OTB_USE_EXTERNAL_LIBKML "Use external LibKML library." ON)
 else()
   option(OTB_USE_EXTERNAL_LIBKML "Use external LibKML library." OFF)
-  set(LIBKML_LIBRARIES otbkml)
-  message(STATUS "Using LibKML internal version")
 endif()
+mark_as_advanced(OTB_USE_EXTERNAL_LIBKML)
 
-# Known issue in OTB which does not compile with external ITK and internal libkml 
-if (OTB_USE_EXTERNAL_ITK AND NOT OTB_USE_EXTERNAL_LIBKML)
-  message(WARNING "There is a known issue which does not allow to use internal libKML with external ITK in OTB."
-                  "You might consider using an external LibKML. "
-                  "See http://bugs.orfeo-toolbox.org/view.php?id=896 for details")
+if(OTB_USE_EXTERNAL_LIBKML)
+  if(NOT LIBKML_FOUND)
+    message(FATAL_ERROR "Can't build OTB without LibKML. Instal it "
+                        "on your system, or disable the option "
+                        "OTB_USE_EXTERNAL_LIBKML to use internal one")
+  endif()
+else()
+  set(LIBKML_LIBRARIES otbkml)
+  message(STATUS "  Using LibKML internal version")
+  # Known issue in OTB which does not compile with external ITK and internal libkml 
+  if (OTB_USE_EXTERNAL_ITK)
+    message(WARNING "There is a known issue which does not allow to use internal libKML with external ITK in OTB."
+                    "You might consider using an external LibKML. "
+                    "See http://bugs.orfeo-toolbox.org/view.php?id=896 for details")
+  endif()
 endif()
-
-mark_as_advanced(OTB_USE_EXTERNAL_LIBKML)