review request (S): 8042255 make gc src file exclusion more automatic

[John Coomes]

John Coomes (John.Coomes at oracle.com) wrote:
> Please review this small makefile change to reduce the need to update
> the list of files in make/excludeSrc.make.
> 
> http://cr.openjdk.java.net/~jcoomes/8042255-gc-exclude/

A couple of people on the gc team have looked at these changes and
don't object, but they also requested that someone with real makefile
expertise take a look.  Anyone here willing to look at it?  Only one
file changed.

-John