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/command.c b/lib/command.c
index 64563b5..3b3fada 100644
--- a/lib/command.c
+++ b/lib/command.c
@@ -84,7 +84,7 @@
 "\r\n\
 Hello, this is " QUAGGA_PROGNAME " (version " QUAGGA_VERSION ").\r\n\
 " QUAGGA_COPYRIGHT "\r\n\
-\r\n";
+" GIT_INFO "\r\n";
 
 
 static const struct facility_map {
@@ -2409,7 +2409,7 @@
 {
   vty_out (vty, "Quagga %s (%s).%s", QUAGGA_VERSION, host.name?host.name:"",
 	   VTY_NEWLINE);
-  vty_out (vty, "%s%s", QUAGGA_COPYRIGHT, VTY_NEWLINE);
+  vty_out (vty, "%s%s%s", QUAGGA_COPYRIGHT, GIT_INFO, VTY_NEWLINE);
 
   return CMD_SUCCESS;
 }