Dynamically build rebar3 if needed

Also tweaked `tz-erl` with some proposed updates from
https://github.com/choptastic/qdate_localtime/pull/1

Makefile

@@ -1,11 +1,24 @@
-all:
+all: rebar3
 	./rebar3 compile
 
-check:
+check: rebar3
 	./rebar3 eunit
 
-dialyzer:
+dialyzer: rebar3
 	./rebar3 dialyzer
 
-publish:
+publish: rebar3
 	./rebar3 hex publish
+
+rebar3:
+	@(echo "Building rebar3...")
+	@(rm -fr tmp)
+	@(mkdir -p tmp)
+	@(cd tmp && \
+		git clone https://github.com/erlang/rebar3 && \
+		cd rebar3 && \
+		./bootstrap)
+	@(echo "Moving rebar3 executable locally (NOT installing system-wide)")
+	@(mv tmp/rebar3/rebar3 .)
+	@(echo "Cleaning up rebar3 remnants")
+	@(rm -fr tmp)