Make: Support shallow git clones for testing

Since commit 1ec9b94, we derive the version number from git tags with
build-aux/git-version-gen.  When a shallow clone doesn't include a
suitable tag, this fails, and make refuses to build anything.  Since
Travis uses git-clone --depth=50, it'll break as soon as we've got
more than 50 commits since the last release.

Support arbitrarily shallow clones for limited purposes like testing
by falling back from a proper V.N-H version number to UNKNOWN-H.

To guard against use of such builds for other purposes, log a warning
on server startup, and print one on player login.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>

src/server/main.c

@@ -31,7 +31,7 @@
  *     Steve McClure, 1996, 1998
  *     Doug Hay, 1998
  *     Ron Koenderink, 2004-2009
- *     Markus Armbruster, 2005-2014
+ *     Markus Armbruster, 2005-2017
  */
 
 #include <config.h>
@@ -380,6 +380,8 @@ start_server(int flags)
     logerror("Empire server (pid %d) started", (int)pid);
     if (running_test_suite)
 	logerror("Configured for testing");
+    else if (strstr(version, "UNKNOWN-"))
+	logerror("Warning: version number unknown");
 
     empth_init((void **)&player, flags);