Makefile.am: Move the lex-rules.am include to the main Makefile

Instead of including the same lex-rules.am from multiple places, include
it from the main Makefile once. This significantly reduces the number of
warnings produced during make.

Also, redefine the .y.c suffix rule to empty, to cut down on warnings
even more.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>