This shows how to do some "computations" in meson.build using its array
and dictionary data structures, and also a basic usage of the sourceset
module for conditional compilation.

Notice the new "if have_system" part of util/meson.build, which fixes
a bug in the old build system was buggy: util/dbus.c was built even for
non-softmmu builds, but the dependency on -lgio was lost when the linking
was done through libqemuutil.a.  Because all of its users required gio
otherwise, the bug was hidden.  Meson instead propagates libqemuutil's
dependencies down to its users, and shows the problem.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>