Change to config(8) for OFED

[Jeff Roberson]

Hi Folks,

I have a patch to permit compilation of files brought in from linux that 
use the same name in multiple directories.  I added a new directive called 
'obj-prefix' which expects a quoted string that will be prepended to the 
object file name.  The problem is that this breaks ${.IMPSRC}

I could also emit the source file name for those rules which are created 
with prefixes and then require each file that uses a prefix to also use 
compile-with.  This wouldn't be so bad since they likely are already using 
compile-with anyway.

Any other suggestions?  Can I override IMPSRC?

Thanks,
Jeff