Add DIALYZER_PLT variable

This is useful for optionally sharing a PLT amongst projects or
dependencies.
Update relx to the latest version
Change erlydtl call to work with old and new releases
Merge branch 'dialyzer-plt-variable' of git://github.com/danielwhite/erlang.mk
Update copyright years
Merge branch 'fix_dtl_compilation' of git://github.com/acammack/erlang.mk
Merge branch 'update_relx' of git://github.com/acammack/erlang.mk
Merge of extend/erlang.mk

Conflicts:
    erlang.mk
switch from `if` condition to using build targets.