internals: Assume compiler supports a subset of ISO varargs syntax

We have considerable anecdotal evidence that every relevant compiler
supports at least the small part of ISO varargs syntax that we need
here, because tools/tool-common.h has contained

    #define VERBOSE(...) do {} while (0)

since dbus 1.9.2 (2014) and nobody has complained yet. With that in
mind, let's simplify.

Signed-off-by: Simon McVittie <smcv@collabora.com>
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