autoconf: Automatically derive version number from latest git tag

with a fallback to get the version string from a .tarball-version file

configure.ac

@@ -1,14 +1,19 @@
 #                                               -*- Autoconf -*-
 # Process this file with autoconf to produce a configure script.
 
-AC_PREREQ(2.68)
-AC_INIT([usbmuxd], [1.1.2], [https://github.com/libimobiledevice/usbmuxd/issues],, [https://libimobiledevice.org])
+AC_PREREQ([2.68])
+AC_INIT([usbmuxd], [m4_esyscmd(./git-version-gen $RELEASE_VERSION)], [https://github.com/libimobiledevice/usbmuxd/issues], [], [https://libimobiledevice.org])
 AM_INIT_AUTOMAKE([dist-bzip2 no-dist-gzip check-news])
 m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES])
 AC_CONFIG_SRCDIR([src/])
 AC_CONFIG_HEADERS([config.h])
 AC_CONFIG_MACRO_DIR([m4])
 
+# Check if we have a version defined
+if test -z $PACKAGE_VERSION; then
+  AC_MSG_ERROR([PACKAGE_VERSION is not defined. Make sure to configure a source tree checked out from git or that .tarball-version is present.])
+fi
+
 # Checks for programs.
 AC_PROG_CC
 AM_PROG_CC_C_O

git-version-gen

@@ -0,0 +1,19 @@
+#!/bin/sh
+SRCDIR=`dirname $0`
+if test -n "$1"; then
+  VER=$1
+else
+  if test -d "${SRCDIR}/.git" && test -x "`which git`" ; then
+    git update-index -q --refresh
+    if ! VER=`git describe --tags --dirty 2>/dev/null`; then
+      COMMIT=`git rev-parse --short HEAD`
+      DIRTY=`git diff --quiet HEAD || echo "-dirty"`
+      VER=`sed -n '1,/RE/s/Version \(.*\)/\1/p' ${SRCDIR}/NEWS`-git-${COMMIT}${DIRTY}
+    fi
+  else
+    if test -f "${SRCDIR}/.tarball-version"; then
+      VER=`cat "${SRCDIR}/.tarball-version"`
+    fi
+  fi
+fi
+printf %s "$VER"