Improve support for OSX/macOS -> Update Makefile so that `sudo make install` works

[jjarava]

Hi!

I've complied the software on macOS (High Sierra) without issues, but make install doesn't work:

jjarava$ make install
install -m 755 -s -o root -g root bchunk		/usr/local/bin
install: root: Invalid argument
make: *** [installbin] Error 67

the proper "group" would be wheel as there's no "root" group in macOS

So the command that the makefile needs to run in macOS is:

install -m 755 -s -o root -g wheel bchunk /usr/local/bin