distgen fails to override --output file with mod 0444

[praiskup]

In Makefiles it is sometimes convenient to make the output file read-only, so users don't mistakenly edit the build product (but rather they edit the template). Distgen though fails to override such files.

Reproducer:

$ cat Makefile 
file: template
        dg --template $< --output $@
        chmod -w $@
$ touch template
$ make
dg --template template --output file
chmod -w file
$ make
make: 'file' is up to date.
$ touch template
$ make
dg --template template --output file
dg: CRITICAL: can't move '/tmp/distgen-890l28un' into 'file'
make: *** [Makefile:2: file] Error 1