kid51/makefile_variables branch

[James E Keenan]

As mentioned in a separate thread on March 18, I have been doing some 
Makefile cage-cleaning in the kid51/makefile_variables branch.  All 
changes are confined to config/gen/makefiles/root.in and most changes 
are the substitution of make variables for hard-coded strings such as 
directory paths.

git diff -w master...kid51/makefile_variables -- 
config/gen/makefiles/root.in

I expect to merge this into master this weekend unless there is some 
objection.

Thank you very much.
Jim Keenan