Make root tool configurable

On Macports one need to run ronn-1.9 instead of ronn. Therefore
make ronn tool configurable. one can now run

make RONN=ronn-1.9

to execute ronn-1.9 instead of the default ronn.

Signed-off-by: Gregor Jasny <gjasny@googlemail.com>
Merge commit 'ccf40b91a67127084ee81e875ee5b77b3b0f019d'

Conflicts:
	Makefile
Merge branch 'master' of github.com:RichiH/vcsh