determine otp version from erlang:system_info(otp_release)

works better for non-standard installations of otp

Author: Andrei Zavada

Makefile

@@ -4,7 +4,7 @@ PKG_REVISION    ?= $(shell git describe --tags 2>/dev/null)
 PKG_BUILD        = 1
 BASE_DIR         = $(shell pwd)
 ERLANG_BIN       = $(shell dirname $(shell which erl 2>/dev/null) 2>/dev/null)
-OTP_VER          = $(shell echo $(ERLANG_BIN) | rev | cut -d "/" -f 2 | rev)
+OTP_VER          = $(shell erl -eval 'erlang:display(erlang:system_info(otp_release)), halt().' -noshell)
 REBAR           ?= $(BASE_DIR)/rebar3
 OVERLAY_VARS    ?=
 TEST_IGNORE     ?= lager riak basho_bench