While building tcltk as an ordinary user I get:

------------------------------------
$rpm --rebuild tcltk-8.0.5-30.src.rpm
[...]
#!/bin/sh
+ cp -a
/var/tmp/tcltk-root/usr/lib/iwidgets2.2.0/demos/buttonbox
/var/tmp/tcltk-root/usr/lib/iwidgets2.2.0/demos/buttonbox.in
+ sed s|/var/tmp/tcltk-root||
/var/tmp/rpm-tmp.30150:
/var/tmp/tcltk-root/usr/lib/iwidgets2.2.0/demos/buttonbox:
Keine Berechtigung
Bad exit status from /var/tmp/rpm-tmp.30150 (%install)

$ ls -l
/var/tmp/tcltk-root/usr/lib/iwidgets2.2.0/demos/buttonbox
-r-xr-xr-x   1 ensc     users         611 Jul 21 02:14
/var/tmp/tcltk-root/usr/lib/iwidgets2.2.0/demos/buttonbox
--------------------------------------

Additionally the owner/group values of the files will remain
the id of the user building the RPM.

I will send a patch to correct the .spec file.