configure.ac: fail when ALSA is enabled but not found

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.

m4/mpd_auto.m4

@@ -0,0 +1,40 @@
+AC_DEFUN([MPD_AUTO_ENABLED], [
+	var="enable_$1"
+	feature="$2"
+
+	if eval "test x`echo '$'$var` = xauto"; then
+		AC_MSG_NOTICE([auto-detected $feature])
+		eval "$var=yes"
+	fi
+])
+
+AC_DEFUN([MPD_AUTO_DISABLED], [
+	var="enable_$1"
+	feature="$2"
+	msg="$3"
+
+	if eval "test x`echo '$'$var` = xauto"; then
+		AC_MSG_WARN([$msg -- disabling $feature])
+		eval "$var=no"
+	else
+		AC_MSG_ERROR([$msg])
+	fi
+])
+
+AC_DEFUN([MPD_AUTO_RESULT], [
+	name="$1"
+	var="enable_$1"
+	found="found_$name"
+	feature="$2"
+	msg="$3"
+
+	if eval "test x`echo '$'$var` = xno"; then
+		eval "$found=no"
+	fi
+
+	if eval "test x`echo '$'$found` = xyes"; then
+                MPD_AUTO_ENABLED([$name], [$feature])
+	else
+                MPD_AUTO_DISABLED([$name], [$feature], [$msg])
+	fi
+])