* +ERLANG_FOOBAR_REBAR_ENV+, to specify additional environment
variables to pass to the _rebar_ utility.
+* +ERLANG_FOOBAR_KEEP_DEPENDENCIES+, to keep the dependencies
+ described in the rebar.config file. Valid values are +YES+ or +NO+
+ (the default). Unless this variable is set to +YES+, the _rebar_
+ infrastructure removes such dependencies in a post-patch hook to
+ ensure rebar does not download nor compile them.
+
With the rebar infrastructure, all the steps required to build
and install the packages are already defined, and they generally work
well for most rebar-based packages. However, when required, it is