target `debian` builds every time

[ppxl]

The debian target does now create the .deb file everytime, even when it was already built and no change happened.

Previously it was bount to the binary which was about to be built. The changes was introduced because there are projects that do not consist of building a binary and yet have to produce a debian package.

Instead the project should target the name the files which lead to the original change so the build target is only called if necessary.

[robertauer]

This problem may be caused by the prepare-package target. If the definition of the PREPARE_PACKAGE variable would not contain a phony target (and would be only filled when defined via the main Makefile) this might fix the problem.