This allows to ignore lines from the default build.config if needed.
For example to not include packages run make WITHOUT=index

If the application's Makefile specify either:
    DEP_PLUGINS = $(PROJECT)
or e.g.:
    DEP_PLUGINS = $(PROJECT)/mk/dist.mk
then load the plugin from the application instead of a dependency.

This helps when you have an application with common Erlang modules and
Erlang.mk plugins: your common application can load Erlang.mk plugins
exactly like other applications depending on the common application.

We do not touch the erlang.mk file anymore. We only patch the
Makefile to include the top-level Erlang.mk by changing the
'include erlang.mk' line to look for ERLANG_MK_FILENAME if it
is defined. The modified Makefile can be committed safely.

Regular plugins (`$(DEP_PLUGINS)`) are loaded near the end of Erlang.mk.
This is fine when you want to modify variables initialized earlier in
Erlang.mk or add new targets and variables.

However, it doesn't allow you to declare more dependencies because they
are loaded too late for that.

This commit introduces a new variable, `$(DEP_EARLY_PLUGINS)`, which can
be used to list plugins meant to be loaded near the beginning of
Erlang.mk. Those allow to append to the list of dependencies.

They work exactly like regular plugins otherwise. The default filename
loaded is `early-plugins.mk`.

we also use LOCAL_DEPS at the top-level, if it exists, to determine
which local apps to compile, in the interest of saving compile time
for projects that have many apps and multiple release configurations

Suggesting -Sy is a terrible idea[1]. This commit replaces it with -S
which is the default on how packages should be added on Archlinux.

Resolves #662.

[1]: https://bbs.archlinux.org/viewtopic.php?id=89328

`ERLC_ASN1_OPTS` can be used to pass compiler options when compiling
ASN.1 files. By default, Erlang.mk will leave this empty, but it can
be redefined in a Makefile.