Makefile: add a PREFIX variable for locations other than /usr.
Makefile: add a PREFIX variable for locations other than /usr.

make install PREFIX=/usr/local

Signed-off-by: Michael Barabanov <191d2c2f6595ea4d754694035f93e083a53fae6d@gmail.com>

tjanez/bup,gevaerts/bup,mhoeher/bup,tjanez/bup,pombredanne/bup,tjanez/bup,mhoeher/bup,mhoeher/bup,apenwarr/bup,gevaerts/bup,jbaber/bup,mhoeher/bup,pombredanne/bup,ToxicFrog/bup,pombredanne/bup,ToxicFrog/bup,ToxicFrog/bup,gevaerts/bup,tjanez/bup,apenwarr/bup,apenwarr/bup,ToxicFrog/bup,mhoeher/bup,jbaber/bup,pombredanne/bup,apenwarr/bup,tjanez/bup,mhoeher/bup,jbaber/bup,jbaber/bup,ToxicFrog/bup,jbaber/bup,gevaerts/bup,tjanez/bup,gevaerts/bup,jbaber/bup,pombredanne/bup,pombredanne/bup