Give more helpful error message when libasound2-dev is missing.

configure.in

@@ -5983,7 +5983,7 @@ if test -n "$MOZ_SYDNEYAUDIO"; then
 linux*)
       PKG_CHECK_MODULES(MOZ_ALSA, alsa, ,
          [echo "$MOZ_ALSA_PKG_ERRORS"
-          AC_MSG_ERROR([Need alsa for Ogg or Wave decoding on Linux.  Disable with --disable-ogg --disable-wave.])])
+          AC_MSG_ERROR([Need alsa for Ogg or Wave decoding on Linux.  Disable with --disable-ogg --disable-wave.  (On Ubuntu, you might try installing the package libasound2-dev.)])])
       ;;
    esac
 fi