For some cases, we cannot decide if a target would change just by looking
at its prequisites, i.e. it's quite likely that it remains the same
even though a prequisite changed. The updated timestamp would cause
a lot of unnecessary recompiles. In this case, we actually generate
a temporary file, compare it to the old file and only the contents
are different do overwrite the old file.

The "update-if-changed" snippet remains always the same, so let's
put it into a macro instead of duplicating it. After that change,
scripts/mkversion_h is so small that we rather put the three remaining
lines directly into the Makefile.