GNU Make: check if a file or directory exists

[Jim]

Hello,

Is there a way I can check myself for the existence of a file or 
directory in make and provide specific error message for my users?

For example if the user calls:
   make build_vxworks

I would like a message that says:
   You have not generated source code for any vxworks targets. Please 
... blah blah blah ...

I hope this makes sense ... I am digging thru examples but not finding 
what i need ...

Cheers,

Jim