[Crash-utility] RFC: crash extension module handling

[Dave Anderson]

Jeff Moyer wrote:

> If you list an explicit target for sial.so, then it will be used.  So,
> you'd explicity add sial.so to the all: line, then add a target for
> sial.so as you normally would.  Does that work for you?
> 
> If you think it will be common to build with different flags or
> libraries, this isn't going to work so well!
> 
> -Jeff

You're right.  I don't think it will be common, but just in
case, I'll still have extensions/Makefile look for all .c files,
but if an accompanying .mk file with the same prefix exists,
run that makefile instead.  Otherwise it will just default
to the standard build line.

Thanks again,
   Dave