Remove hardcoded paths

[rski]

See https://github.com/6WIND/zrpcd/blob/master/zrpcd/zrpc_vpnservice.c#L220 and https://github.com/6WIND/zrpcd/blob/master/zrpcd/zrpc_vpnservice.h#L24

These should be configurable somehow, because otherwise packagers have to patch the source code. I think autotools have something that can help with this (libtool/config.h?)

[pguibert6WIND]

Currently, path is hardcoded, and there is a need to patch zrpc according to quagga path.

I would say that path of quagga should be an option to configure in zrpc configure.