build: include git info

If enabled with --with-pkg-gitversion on ./configure, this will append
git version strings and branch information at the following places:
 - overall version number: 0.99.21-g0123456
 - login motd and show version: tag information + git id + branches

Sample output:
  Hello, this is Quagga (version 0.99.21-g14b49ad-dirty).
  Copyright 1996-2005 Kunihiro Ishiguro, et al.

  This is a git build of quagga_0_99_21_release-106-g14b49ad-dirty
  Associated branch(es):
	local:master

[v2]: fix build without gitinfo (add "else" branch)
[v2]: fix for repos without any tags (different git describe output)
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/lib/.gitignore b/lib/.gitignore
index 00af85a..02aa432 100644
--- a/lib/.gitignore
+++ b/lib/.gitignore
@@ -5,6 +5,8 @@
 *.la
 version.c
 version.h
+gitversion.h
+gitversion.h.tmp
 .deps
 .nfs*
 .libs