Best practice to handle custom build packages with import_package

[jakobs]

The custom package building documentation here
https://www.rock-robotics.org/documentation/autoproj/advanced/autobuild.html
is a bit thin, and I was wondering if there was a best practice on how to handle custom packages.
There are many packages out there that just use a custom build script. Mostly make based anyway. Or something which looks like autotools but isn't (e.g. qt5).