Cleanup Makefile.

Last pull request with minor cleanups for the day - I promise!

---

Stopped using non-standard rm(1) flags: While these worked on some distribtions, being non-standard, they simply fail on others, and we can work around them anyway.
Noticably, this fixes installation on BSDs and other non-GNU *nix distributions (eg: OpenBSD, FreeBSD).

Use install(1) instead of cp(1) and mkdir(1), since it's cleaner (ie: easier to read), shorter and makes sure permissions are what we want the to be is standard in makefiles.