build: Add target install and install-strip.

It has always been a bit clumsy to copy the executable and the
manual page to their proper locations by hand, so this commit adds
the two standard targets "install" and "install-strip" which install
both files.

The installation prefix defaults to /usr/local and can be set with
PREFIX. Moreover, there is DESTDIR which may be given to prepend
another directory (useful for for "staged installs", where the
installed files are not placed directly into their expected location
but are instead copied into a temporary location).