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/Makefile.am b/lib/Makefile.am
index e00ad54..690c18f 100644
--- a/lib/Makefile.am
+++ b/lib/Makefile.am
@@ -14,7 +14,7 @@
 	zclient.c sockopt.c smux.c agentx.c snmp.c md5.c if_rmap.c keychain.c privs.c \
 	sigevent.c pqueue.c jhash.c memtypes.c workqueue.c
 
-BUILT_SOURCES = memtypes.h route_types.h
+BUILT_SOURCES = memtypes.h route_types.h gitversion.h
 
 libzebra_la_DEPENDENCIES = @LIB_REGEX@
 
@@ -36,3 +36,25 @@
 
 route_types.h: $(srcdir)/route_types.txt $(srcdir)/route_types.pl
 	@PERL@ $(srcdir)/route_types.pl < $(srcdir)/route_types.txt > $@
+
+if GIT_VERSION
+
+# bit of a trick here to always have up-to-date git stamps without triggering
+# unneccessary rebuilds.  .PHONY causes the .tmp file to be rebuilt always,
+# but if we use that on gitversion.h it'll ripple through the .c file deps.
+# (even if gitversion.h's file timestamp doesn't change, make will think it
+# did, because of .PHONY...)
+
+.PHONY: gitversion.h.tmp
+.SILENT: gitversion.h gitversion.h.tmp
+GITH=gitversion.h
+gitversion.h.tmp: $(srcdir)/../.git
+	@PERL@ $(srcdir)/gitversion.pl $(srcdir) > ${GITH}.tmp
+gitversion.h: gitversion.h.tmp
+	{ test -f ${GITH} && diff -s -q ${GITH}.tmp ${GITH}; } || cp -v ${GITH}.tmp ${GITH}
+
+else
+.PHONY: gitversion.h
+gitversion.h:
+	/bin/true
+endif