Build: release.mk: Fix the fallback setting of VERSION.

Previously, it was something like "Pacemaker-1.2.3". This would lead to
creating srpm files named "pacemaker-Pacemaker-1.2.3.src.rpm".
Additionally, the configure-generated VERSION did not include the
"Pacemaker-" suffix.

Now, it's just the version number.

Ref T912