Support Elixir as a dependency via ELIXIR_PATCH=1

For example:

DEPS = elixir
ELIXIR_PATCH = 1
include erlang.mk

A language test suite will soon be added with a test for this.

core/deps.mk

@@ -665,6 +665,8 @@ ifeq ($(filter $(1),$(NO_AUTOPATCH)),)
 			echo " PATCH  Downloading rabbitmq-codegen"; \
 			git clone https://github.com/rabbitmq/rabbitmq-codegen.git $(DEPS_DIR)/rabbitmq-codegen; \
 		fi \
+	elif [ "$1" = "elixir" -a "$(ELIXIR_PATCH)" ]; then \
+		ln -s lib/elixir/ebin $(DEPS_DIR)/elixir/; \
 	else \
 		$$(call dep_autopatch,$(DEP_NAME)) \
 	fi

index/elixir.mk

@@ -0,0 +1,7 @@
+PACKAGES += elixir
+pkg_elixir_name = elixir
+pkg_elixir_description = Elixir is a dynamic, functional language designed for building scalable and maintainable applications
+pkg_elixir_homepage = https://elixir-lang.org/
+pkg_elixir_fetch = git
+pkg_elixir_repo = https://github.com/elixir-lang/elixir
+pkg_elixir_commit = master