Makefile: use automake rule for compiling .S

Automake seems to have its own rules for compiling an .o from an .S.
Essentially it does the same as our hand-crafted rule, but adds some
things like dependency file generation.

Remove our hand-crafted rule to use the automake rule, it is less
surprising.

http://www.gnu.org/software/automake/manual/html_node/Assembly-Support.html



Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Tested-by: Víctor Jáquez <vjaquez@igalia.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>