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

CMake/ImportTinyXML.cmake

@@ -3,19 +3,27 @@ find_package(TinyXML)
 
 if(TINYXML_FOUND)
   option(OTB_USE_EXTERNAL_TINYXML "Use external TinyXML library." ON)
-
-  # Check if STL support is enabled with macro TIXML_USE_STL
-  try_compile(TIXML_USE_STL
-              ${CMAKE_CURRENT_BINARY_DIR}/CMake
-              ${CMAKE_CURRENT_SOURCE_DIR}/CMake/otbTestTinyXMLUseSTL.cxx
-              CMAKE_FLAGS "-DINCLUDE_DIRECTORIES:PATH=${TINYXML_INCLUDE_DIRS}" "-DLINK_LIBRARIES:STRING=${TINYXML_LIBRARIES}"
-              COMPILE_DEFINITIONS "-DTIXML_USE_STL"
-              OUTPUT_VARIABLE OUTPUT)
-
 else()
   option(OTB_USE_EXTERNAL_TINYXML "Use external TinyXML library." OFF)
+endif()
+mark_as_advanced(OTB_USE_EXTERNAL_TINYXML)
+
+if(OTB_USE_EXTERNAL_TINYXML)
+  if(TINYXML_FOUND)
+    # Check if STL support is enabled with macro TIXML_USE_STL
+    try_compile(TIXML_USE_STL
+                ${CMAKE_CURRENT_BINARY_DIR}/CMake
+                ${CMAKE_CURRENT_SOURCE_DIR}/CMake/otbTestTinyXMLUseSTL.cxx
+                CMAKE_FLAGS "-DINCLUDE_DIRECTORIES:PATH=${TINYXML_INCLUDE_DIRS}" "-DLINK_LIBRARIES:STRING=${TINYXML_LIBRARIES}"
+                COMPILE_DEFINITIONS "-DTIXML_USE_STL"
+                OUTPUT_VARIABLE OUTPUT)
+  else()
+    message(FATAL_ERROR "Can't build OTB without TinyXML. Install it "
+                        "on your system, or disable the option "
+                        "OTB_USE_EXTERNAL_TINYXML to use the internal one")
+  endif()
+else()
   set(TINYXML_LIBRARIES otbtinyXML)
   set(TIXML_USE_STL)
-  message(STATUS "Using TinyXML internal version")
+  message(STATUS "  Using TinyXML internal version")
 endif()
-mark_as_advanced(OTB_USE_EXTERNAL_TINYXML)