Update CMakeLists.txt

Add explicit linking of irrxml.

test/CMakeLists.txt

@@ -207,7 +207,7 @@ SET_PROPERTY( TARGET assimp PROPERTY DEBUG_POSTFIX ${CMAKE_DEBUG_POSTFIX} )
 IF( WIN32 )
     SET( platform_libs )
 ELSE()
-    SET( platform_libs pthread )
+    SET( platform_libs pthread irrXML )
 ENDIF()
 
 IF(MSVC)