[mythtv] New configure --prefix is not updating settings.pro

[Sasha Z]

Is it just me, or does "./configure --prefix <path>" not work? It
changes the setting in config.mak, and even though that file is
imported by settings.pro/mythplugins.pro, it does not have an effect,
and the path becomes /usr/local anyway. I'm making packages for Arch
linux, and I'm adding sed lines all over the place to fix the .pro
files before compile. Is there something stupid that I have
overlooked?