make: Don't build dist for run-broker by default

During development we want `make run-broker` to execute fast,
yet still pick up the changes we made in rabbit applications.
We can already do this by setting the appropriate variables.
This commit makes it so that this is the default. Now instead
of depending on the `dist` target we run plugins from the deps/
directory. And we depend on `all` to pick up changes.

This is equivalent to running
`make run-broker PLUGINS_FROM_DEPS_DIR=1 DIST_TARGET=all`.

It can be disabled by setting `FAST_RUN_BROKER=0`.

It doesn't invalide the `NOBUILD=1` variable which lets us
run the broker without recompiling (used in tests). It also
doesn't make `NOBUILD=1` faster (or slower).

The difference when running `make run-broker` by default is
roughly half the time of what it was before:

  make run-broker  16,67s user 10,42s system 101% cpu 26,567 total
  make run-broker  8,75s user 4,40s system 102% cpu 12,873 total

And it also applies to `make start-cluster`:

  make start-cluster  26,32s user 15,15s system 141% cpu 29,279 total
  make start-cluster  18,09s user 8,76s system 170% cpu 15,726 total

Author: Loïc Hoguin

deps/rabbit_common/mk/rabbitmq-run.mk

@@ -96,16 +96,22 @@ RABBITMQ_ENABLED_PLUGINS_FILE ?= $(call node_enabled_plugins_file,$(RABBITMQ_NOD
 RABBITMQ_LOG ?= debug,+color
 export RABBITMQ_LOG
 
-# erlang.mk adds dependencies' ebin directory to ERL_LIBS. This is
-# a sane default, but we prefer to rely on the .ez archives in the
-# `plugins` directory so the plugin code is executed. The `plugins`
-# directory is added to ERL_LIBS by rabbitmq-env.
-DIST_ERL_LIBS = $(patsubst :%,%,$(patsubst %:,%,$(subst :$(APPS_DIR):,:,$(subst :$(DEPS_DIR):,:,:$(ERL_LIBS):))))
+FAST_RUN_BROKER ?= 1
+
+ifeq ($(FAST_RUN_BROKER),1)
+DIST_TARGET = $(if $(NOBUILD),,all)
+PLUGINS_FROM_DEPS_DIR = 1
+endif
 
 ifdef PLUGINS_FROM_DEPS_DIR
-RMQ_PLUGINS_DIR=$(DEPS_DIR)
+RMQ_PLUGINS_DIR = $(DEPS_DIR)
+DIST_ERL_LIBS = $(ERL_LIBS)
 else
-RMQ_PLUGINS_DIR=$(CURDIR)/$(DIST_DIR)
+RMQ_PLUGINS_DIR = $(CURDIR)/$(DIST_DIR)
+# We do not want to add apps/ or deps/ to ERL_LIBS
+# when running the release from dist. The `plugins`
+# directory is added to ERL_LIBS by rabbitmq-env.
+DIST_ERL_LIBS = $(patsubst :%,%,$(patsubst %:,%,$(subst :$(APPS_DIR):,:,$(subst :$(DEPS_DIR):,:,:$(ERL_LIBS):))))
 endif
 
 node_plugins_dir = $(if $(RABBITMQ_PLUGINS_DIR),$(RABBITMQ_PLUGINS_DIR),$(if $(EXTRA_PLUGINS_DIR),$(EXTRA_PLUGINS_DIR):$(RMQ_PLUGINS_DIR),$(RMQ_PLUGINS_DIR)))