feat: allow showing compilation info via env var

[rNoz]

This PR allows using an env var ERLEXEC_REBAR_COMPILE_INFO to execute both Makefile targets info and all (aka compile) at once, helpful when being compiled from Elixir. Therefore, in case errors like priv/false* appear in the future, we can track what was the compilation log, since it just emits 6 lines and can be beneficial to understand the origin of the trouble. In the end, it is optional.

• "priv/false" or any error like :failed_to_start_child, :exec, {:bad_return_value... indicating the exec-port is not found/available.

For instance, two examples when being executed from Elixir toolchain.

a) while building normally:

$ mix do deps.compile erlexec, compile
make: Nothing to be done for `all'.
===> Analyzing applications...
===> Compiling erlexec
==> project
Generated project app

b) using this feature:

$ ERLEXEC_REBAR_COMPILE_INFO=1 mix do deps.compile erlexec, compile
SOURCES: ei++.cpp exec_impl.cpp exec.cpp
OBJECTS: ei++.o exec_impl.o exec.o
TARGET:  aarch64-apple-darwin
MARCH:   aarch64-apple-darwin
OUTPUT:  /Users/user/project/deps/erlexec/priv/aarch64-apple-darwin/exec-port
OUT_DIR: /Users/user/project/deps/erlexec/priv/aarch64-apple-darwin/
make: Nothing to be done for `all'.
===> Analyzing applications...
===> Compiling erlexec
==> project
Generated project app

[saleyn]

I added this functionality using a more compact implementstion.