Use release helper from libosmocore

See
https://osmocom.org/projects/cellular-infrastructure/wiki/Make_a_new_release
for details.

Change-Id: Ieb843923d8f534654413be695f2b5f0c87b75520
Related: OS#1861
diff --git a/configure.ac b/configure.ac
index 8cfd25d..43b5403 100644
--- a/configure.ac
+++ b/configure.ac
@@ -34,6 +34,10 @@
 dnl Linux kernel KBuild style compile messages
 m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])
 
+dnl include release helper
+RELMAKE='-include osmo-release.mk'
+AC_SUBST([RELMAKE])
+
 AM_PROG_AS
 AC_PROG_CXX
 AX_CXX_COMPILE_STDCXX_11