allow prefix to be overridden on the command-line

[eddelbuettel]

Currently Makefile hard-code prefix to /usr/local which is incompatible with how distributions package content. Switching the assignment to ?= lets the current behaviour (of using /usr/local by default) to continue yet permits setting an override.

A simple demo using a (not-committed) make target that just echos the value:

edd@rob:~/git/xrprof(master)$ make -n showprefix
echo "Prefix is /usr/local"
edd@rob:~/git/xrprof(master)$ make -n showprefix prefix=/tmp
echo "Prefix is /tmp"
edd@rob:~/git/xrprof(master)$