Use release helper from libosmocore

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

Change-Id: I3ccc8202b650268cc9d1721719ba2f205c351a67
Related: OS#1861
diff --git a/configure.ac b/configure.ac
index ab7a637..72cbb56 100644
--- a/configure.ac
+++ b/configure.ac
@@ -16,6 +16,10 @@
 dnl kernel style compile messages
 m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])
 
+dnl include release helper
+RELMAKE='-include osmo-release.mk'
+AC_SUBST([RELMAKE])
+
 dnl checks for programs
 AC_PROG_MAKE_SET
 AC_PROG_MKDIR_P