Add RMQ_NON_DETERMINISTIC_BUILD make variable

Follow-up to:
* #3442
* 93db480
* #13899
* #14326

Simply changing `+=` to `?=` doesn't work as there are other important
statements added to `RMQ_ERLC_OPTS` that can't be left out when `make
RMQ_ERLC_OPTS=''` is run. This only became apparent when a completely
cleaned-out checkout of `rabbitmq/rabbitmq-server` was attempted to be
built with that command.

This adds `RMQ_NON_DETERMINISTIC_BUILD` which will be used like this:

```
make RMQ_NON_DETERMINISTIC_BUILD=true
```

...and `+deterministic` won't be passed to `erlc` or other commands.

Author: lukebakken

deps/rabbit/Makefile

@@ -367,7 +367,9 @@ RMQ_ERLC_OPTS += -DTRACE_SUPERVISOR2=true
 endif
 
 # https://www.erlang.org/doc/apps/parsetools/leex.html#file/2
-YRL_ERLC_OPTS ?= +deterministic
+ifeq ($(RMQ_NON_DETERMINISTIC_BUILD),)
+YRL_ERLC_OPTS += +deterministic
+endif
 
 # --------------------------------------------------------------------
 # Documentation.

deps/rabbit_common/mk/rabbitmq-build.mk

@@ -15,7 +15,9 @@ ifneq ($(filter rabbitmq_cli,$(BUILD_DEPS) $(DEPS)),)
 RMQ_ERLC_OPTS += -pa $(DEPS_DIR)/rabbitmq_cli/ebin
 endif
 
-RMQ_ERLC_OPTS ?= +deterministic
+ifeq ($(RMQ_NON_DETERMINISTIC_BUILD),)
+RMQ_ERLC_OPTS += +deterministic
+endif
 
 # Push our compilation options to both the normal and test ERLC_OPTS.
 ERLC_OPTS += $(RMQ_ERLC_OPTS)