#7 support DESTDIR

[Mike Frysinger]

the convention in the autotool world is to install in another tree via DESTDIR:
make install DESTDIR=/some/place

currently, ROOTDIR exists that does this same thing ... i dont know if you care about backwards compatibility, but either ROOTDIR can be changed completely to DESTDIR, or you can do something like:
ROOTDIR = $(DESTDIR)
and then both methods work fine