Update Makefile: Do not try to install on "make" or "make all"

To be consistent with usual makefile usage and to not surprise people too much, only do permanent changes to the system when explicitely prompted to do so via "make install" instead of on any "make" or "make all" invocation, which is usually used for *building*, not *installing* things.
Merge pull request #219 from jaseg/patch-1

Update Makefile: Do not try to install on "make" or "make all"