I've been thinking (shock!), and it seems to me that it wouldn't break
anything majorly if we did expand all the $prefix-dependant variables. The
reason being, practically everyone uses automake these days, and automake
defines all installation paths to be prefixed by $DESTDIR, which is empty
by default.
Since the only reason I've been given to keep the variables unexpanded is
so that you can do:
./configure --prefix=/usr
make
make prefix=/tmp/usr install
cd /tmp
tar cfz binary-tarball.tgz usr
this seems to me to be a bit of a redundant feature.