Support $(PREFIX) install target directory prefix

This change introduces support for the common PREFIX variable in the Makefile and install.py, instead of having /usr/local hardcoded. This makes it much easier to install node to custom locations e.g. in a user's home directory.

The PREFIX variable defaults to /usr/local.

The change in install.py to introduce the install_path variable is probably not strictly necessary, but I thought it made it somewhat easier to separate the DESTDIR and PREFIX semantics. If you disagree I can of course update the patch.

See also #4551.