When MPD_AUTO_DISABLED prints a fatal error message, include the
feature name. This might be an important piece of information for the
user, just in case the preceding line doesn't tell him.
This patch adds a small autoconf M4 library which deals with
auto-detected features. The default for those features is "auto",
which is like the old default: if the library is present on the
system, enable the feature, disable otherwise. If the user explicitly
enables that feature (--enable-alsa), and the library is not present,
configure must fail, because it cannot fulfill the request.
