Modification of a patch from Debian: eliminate the endless rebuilds

Modification of a patch from Debian: eliminate the endless rebuilds
of basically unchanged binaries due to the compulsory rebuild of
version.o. version.o now depends normally on version2.def, which is
constructed using much the same if statements that version.o used to
use, except that it's not overwritten at all if its contents don't
need to change.

[originally from svn r8931]