make the bootstrap script recognize automake 1.16

[gizmotronic]

The build-auto.sh script in libdvbtee references this repository, but we can't build it on a system with automake-1.16 installed because the bootstrap script only recognizes versions up to 1.15. While the version of this library on videolan.org does support the newer automake, I wasn't able to build libdvdtee against it (at least, not linked statically). Hence, this pull request.