Fix cross compiling system includes (#45)

Author: woodfell

GenericFindDependency.cmake

@@ -347,9 +347,7 @@ function(GenericFindDependency)
   # Final validation that the target was properly created from some source
   if(TARGET ${x_TARGET})
     if(x_SYSTEM_INCLUDES)
-      if(NOT CMAKE_CROSSCOMPILING OR THIRD_PARTY_INCLUDES_AS_SYSTEM)
-        mark_target_as_system_includes(${x_TARGET})
-      endif()
+      mark_target_as_system_includes(${x_TARGET})
     endif()
   else()
     # Target not found in any location