Add release target to Makefile

Add simple helper target to automate basic release steps:
* version bump
* prepare release commit
* git commit, tag and sign

For library projects:
* update debian/changelog from TODO-RELEASE
* cleanup TODO-RELEASE

For non-library projects:
* update debian/changelog from git log

Note: it requires bumpversion package to be installed, debian/control is
adjusted accordingly. The helper itself is installed to facilitate reuse
by other libraries.

N. B: you still have to manually adjust LIBVERSION in previous commit -
see TODO-RELEASE header for details.

Use it as follows:
make REL=minor release

The REL parameter defines which component of the version [1] to bump and
can be any of { major, minor, patch }.

[1] http://semver.org/

Change-Id: I790ceb958195b9f6cbabfe8c977dc30e2bd7414b
Related: OS#1861
diff --git a/configure.ac b/configure.ac
index 96757df..5821ad3 100644
--- a/configure.ac
+++ b/configure.ac
@@ -11,6 +11,10 @@
 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