Fix some Makefile issues

Four changes I had to make to integrate OSM-binary into my project:
* Allow overriding Makefile variables from the command line
* Allow overriding protoc path from the command line
* Install to /usr/local per default
* Do not set file user:group to 'root:root' during installation

The first three items should be self-explanatory.

The reason for #4 is when 'make install' is run as root, files will be owned by 'root:root' anyway. When run as user changing user:group will fail. Furthermore, the group 'root' is not available on all systems.

Thanks!
Thomas