configure.ac: Use git-version-gen to set VERSION

We updated to 0.9.29 tag, but configure.ac was locked to 0.9.28, which
means release 0.9.29 is going to generated an old version and thus is
broken. A new release will follow this commit.

Change-Id: I8e1665f21d1bc76922423cabad3df72a2502f559
diff --git a/configure.ac b/configure.ac
index ab63045..53720b9 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1,4 +1,5 @@
-AC_INIT([libasn1c], [0.9.28],
+AC_INIT([libasn1c],
+        m4_esyscmd([./git-version-gen .tarball-version]),
         [openbsc-devel@lists.openbsc.org])
 
 AM_INIT_AUTOMAKE([dist-bzip2])
@@ -75,4 +76,3 @@
         include/Makefile
         include/asn1c/Makefile
         Makefile)
-