Pick up date from SOURCE_DATE_EPOCH, for reproducible builds

The goal of reproducible builds is that a rebuild of the same source
code with the same compiler, libraries, etc. should result in the same
binaries. SOURCE_DATE_EPOCH provides a standard way for build systems
to fill in the date of the latest source change, typically from a git
commit or from metadata like the debian/changelog in Debian packages.

This does not change anything if SOURCE_DATE_EPOCH is not defined;
the intention is that a larger build system like a Debian package
will define it.

Please see https://reproducible-builds.org/ for more information about
reproducible builds.

code/game/g_main.c

@@ -101,7 +101,7 @@ static cvarTable_t		gameCvarTable[] = {
 
 	// noset vars
 	{ NULL, "gamename", GAMEVERSION , CVAR_SERVERINFO | CVAR_ROM, 0, qfalse  },
-	{ NULL, "gamedate", __DATE__ , CVAR_ROM, 0, qfalse  },
+	{ NULL, "gamedate", PRODUCT_DATE , CVAR_ROM, 0, qfalse  },
 	{ &g_restarted, "g_restarted", "0", CVAR_ROM, 0, qfalse  },
 
 	// latched vars
@@ -408,7 +408,7 @@ void G_InitGame( int levelTime, int randomSeed, int restart ) {
 
 	G_Printf ("------- Game Initialization -------\n");
 	G_Printf ("gamename: %s\n", GAMEVERSION);
-	G_Printf ("gamedate: %s\n", __DATE__);
+	G_Printf ("gamedate: %s\n", PRODUCT_DATE);
 
 	srand( randomSeed );