make PREFIX work like expected, and use DESTDIR for that

[comotion]

PREFIX is usually used to indicate installation in /, /usr or /usr/local, while DESTDIR is used when creating packages etc. This is how autocrap does it and a lot of packagers expect this behavior.

[lukego]

Thanks! Having a nonstandard make install behavior is unfortunate.

One more alternative we should consider: should we drop our install target? I do like the idea of Snabb Switch being a single file that could be (but needn't be) distributed as a binary. If we would want to create symlinks, manpages, etc then this could perhaps be done by a command snabb install [dest] rather than a Makefile.

[eugeneia]

Assuming make install works otherwise, I think its safe to accept this patch for now and possibly replace make install in another PR. Merged into max-next.