Fix Makefile $(REBAR) variable handling on Illumos

On solaris flavors, `which` has: sometimes poor behavior of
not returning proper exit codes or printing errors to STDOUT
rather than STDERR.  All of this is to say, the makefile didn't
behave correctly on Solaris. $(REBAR) ended up being set to
"rebar not found" which made the compile target less than happy.

I added a print-* target to test this easier.  Simply
`make print-VAR` to see the value.  So `make print-REBAR`
will show you the value of $REBAR.

Makefile

@@ -1,4 +1,4 @@
-REBAR=$(shell which rebar || echo ./rebar)
+REBAR = $(shell test -e `which rebar` 2>/dev/null && which rebar || echo "./rebar")
 DEPSOLVER_PLT=$(CURDIR)/.depsolver_plt
 
 all: dirs compile
@@ -43,3 +43,5 @@ typer: $(DEPSOLVER_PLT)
 
 distclean: clean
 	@rm $(DEPSOLVER_PLT)
+
+print-%: ; @echo $*=$($*)