package/sdl_mixer: fix build after aclocal include revamp

package/sdl_mixer: fix build after aclocal include revamp

After d255b67972 (autotools: do not overwrite first include path), the
ordering of include paths has changed: the system directories are
specified with explicit options passed to autoreconf, which means that
any directory specified in the package _AUTORECONF_OPTS are no longer
first:

  - in package/autoconf/autoconf.mk, we define AUTORECONF as:
    AUTOCONF = $(HOST_DIR)/bin/autoconf -I "$(ACLOCAL_DIR)" -I "$(ACLOCAL_HOST_DIR)"

  - in package/pkg-autotools.mk, we call AUTORECONF with:
    $($(PKG)_AUTORECONF_ENV) $(AUTORECONF) $($(PKG)_AUTORECONF_OPTS)

So, the include directory specified by SDL_MIXER_AUTORECONF_OPTS is now
lagging behind the system headers, and the very issue that d255b67972
was suposed to fix in a generic way, pops up back for this specific
case.

We fix that by patching sdl_mixer so that it uses the bog-down standard
mechanisms, to specify the macro directory from within configure.in,
instead of specifying it on the command line, so that the magic
introduced by d255b67972 does happen.

Reported-by: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Michael Walle <michael@walle.cc>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>