Makefile: respect PREFIX variable

This introduces the standard prefix, exec_prefix, bindir, and libdir
variables according to the GNU Coding Standards:

  http://www.gnu.org/prep/standards/html_node/Directory-Variables.html

The default for prefix is PREFIX and the default for that variable is
now "/usr/local". The implicit value was "/usr" before.
Merge pull request #14 from mgeisler/makefile-prefix

Makefile: respect PREFIX variable