Installation guide: building and plugins Makefile template

[tkapias]

The examples in the installation guide (doc/install.md) will install the config files in /usr/local/etc/dillo.

But most other tutorials and the Makefile template in every official plugins expect /etc/dillo.

The examples should be ../configure --prefix=/usr/local --sysconfdir=/etc.

[rodarima]

The examples should be ../configure --prefix=/usr/local --sysconfdir=/etc.

Plugins should accept non-standard installation paths for /etc (including others than /usr/local/etc), so we don't need to pollute /etc with non-packaged files.

We can search in /etc and /use/local/etc for dpidrc by default for now.

Maybe also add a make variable so we can specify where to look for dpidrc to support other installation prefixes.