Bug#1086702: alternative hello build dependencies

[Matthias Klose]

 From my point of view, these alternative hello dependencies are a hack.

My understanding is that Sylvestre wants to use the very same control 
file for Debian builds, and for his uploads to apt.llvm.org, for 
multiple releases of Debian and Ubuntu.  I don't think that's 
reasonable, but I seem to be in disagreement with Sylvestre.

My proposed fix for that issue is to remove these alternative build 
dependencies, and regenerate the control file before building the 
package. This way, you can get exactly the needed build dependency.

This would be a no-change for the Debian builds, but would require 
changes in the CI how packages are built for apt.llvm.org.  It's fine to 
build such packages out of the Debian repository, but that should not 
become a burden for the Debian packages itself.

The changed CI for apt.llvm.org should do instead of a simple sbuild an 
unpack of the sources, running debian/rules stamps/preconfigure, 
debian/rules clean, and then calling sbuild to get the proper build 
dependencies for each specific build.

Matthias