[krbdev.mit.edu #8356] Change default prefix to /usr/local/

[Karl-Philipp Richter via RT]

Running `./configure` without arguments causes the installation prefix
to be `/usr/` which doesn't comply the widley-used de-facto standard and
the Filesystem Hierarchy Standard afaik. It overwrites binaries
installed by package management systems, like `apt` on popular
Debian-based OS which is unnecessary trouble - and the reason why
de-facto standards or FHS exist.

experienced with 25c651a on Ubuntu 15.10 with Linux 4.3.3

-Kalle