Launching a command like "kdesrc-build --src-only kde/applications" will
update the appropriate modules but never show any output until a timeout
occurs, and then show all output at once.

This is due to the handle_build() method refusing to do anything when
the list of modules to build is empty (as it will be under --src-only).
However handle_build() is what actually processes and shows messages
from the update process during an 'async' build.

Apparently I introduced this bug while refactoring this code a few
months back.  Previously kdesrc-build would have launched in the
'no-async' mode automatically with --src-only or --build-only in effect.

By launching in 'async' mode the various logging codes in the update
process feed all messages to a hidden IPC object which the build process
is expected to read out and show.

We can fix the bug either by making handle_build() always try to do
something (so that it shows messages even if nothing is being built) or
by launching in no-async if there will be only a single type of build
phase (i.e. update or non-update but not both types).  I chose the
latter.