erlc.mk: Use double-colon targets for source files targets
erlc.mk: Use double-colon targets for source files targets

When files are generated, this avoids a warning because the target would
be redefined.

ingwinlu/erlang.mk,KrzysiekJ/erlang.mk,ninenines/erlang.mk,hairyhum/erlang.mk,rabbitmq/erlang.mk,jj1bdx/erlang.mk,a12n/erlang.mk,bsmr-erlang/erlang.mk,crownedgrouse/erlang.mk,nevar/erlang.mk