* ./configure.ac.in: complain when ALSA headers are not found but

    --enable-alsa was specified.

configure.ac.in

@@ -2138,6 +2138,8 @@ AC_ARG_ENABLE(alsa,
      then
        PLUGINS="${PLUGINS} alsa"
        LDFLAGS_alsa="${LDFLAGS_alsa} -lasound -lm -ldl"
+     else
+       AC_MSG_ERROR([Could not find ALSA development headers])
      fi
    fi])