Added -IM4 to autoreconf for libsndfile.

On ubuntu 12.04 libsndfile fails to compile with the following error:

    Failed to build package libsndfile!
    ------------------------------------------------------------
        See the Autoconf documentation.
    configure.ac:291: error: possibly undefined macro: AC_OCTAVE_BUILD
    configure.ac:311: error: possibly undefined macro: PKG_CHECK_MOD_VERSION
    autoreconf: /usr/bin/autoconf failed with exit status: 1
    make[1]: *** [build-only-libsndfile_i686-w64-mingw32.static] Error 1
    make[1]: Leaving directory `/home/eivind/git/mxe'
    real	0m8.879s
    user	0m6.957s
    sys	0m1.097s

It seems that autoreconf need to include the M4 directory to expand the two macros correctly.