Comment 10 for bug 633681

One last note ...

Re-reading the build log in comment #1 ... the last commands displayed are
copying %doc files. Which means that all of what rpmbuild traditionally
foes with helper scripts has already been accomplished.

Note "traditionally".

Hint:
    grep rpmfcExec build/files.c
The checkfiles script is the only helper invocation after %doc is copied.

Try draining stdin always, not just when the $RPM_BUILD_ROOT dir exists:

if [ ! -d "$RPM_BUILD_ROOT" ] ; then
        cat > /dev/null
        exit 1
fi