Cycles: Fix compilation error when OIIO is compiled with external PugiXML parser

Basic idea is to check whether OIIO is compiled with embedded PugiXML parser
and if so use PugiXML from OIIO, otherwise find a standalone PugiXML library.

intern/cycles/app/CMakeLists.txt

@@ -32,6 +32,10 @@ set(LIBRARIES
 	extern_cuew
 )
 
+if(NOT PUGIXML_LIBRARIES STREQUAL "")
+	list(APPEND LIBRARIES ${PUGIXML_LIBRARIES})
+endif()
+
 if(NOT CYCLES_STANDALONE_REPOSITORY)
 	list(APPEND LIBRARIES bf_intern_glew_mx)
 endif()