Description of Problem:
Function 'rpm-build' instead of using start-process should really set
'compilation-command' variable and run 'compile'. The real difference is how you
are walking through errors in a compilation buffer as opposed to "Fundamental"
one. Run "M-x compile" and replace a default compilation commmand by
something like "rpmbuild -ba my.spec" to see for yourself.
This will also likely simplify rpm-spec-mode.el a small bit.
rpm-spec-mode.el maintainer said he'd try to add this feature
in a coming version.