sdl_mixer: add patch to fix SDL_mixer.pc for static linking

sdl_mixer: add patch to fix SDL_mixer.pc for static linking

A 'Libs.private' field needs to be present and properly defined in the
pkg-config file in order for static linking against SDL_mixer to work.

Besides adding the mentioned field, the included patch also modifies
configure.in so that EXTRA_LDFLAGS (which is now also used as the value
of 'Libs.private') no longer includes SDL_LIBS. This is done so as to
prevent libraries required by SDL from being listed twice when
'pkg-config --libs --static SDL_mixer' is run (they're already shown
because of the 'Requires: sdl' line in SDL_mixer.pc). Makefile.in is
adjusted accordingly as well.

Also enable autoreconf for this package since we're patching
configure.in.

Signed-off-by: Rodrigo Rebello <rprebello@gmail.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>