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/sys/sys_main.c

@@ -553,7 +553,7 @@ void Sys_ParseArgs( int argc, char **argv )
 		if( !strcmp( argv[1], "--version" ) ||
 				!strcmp( argv[1], "-v" ) )
 		{
-			const char* date = __DATE__;
+			const char* date = PRODUCT_DATE;
 #ifdef DEDICATED
 			fprintf( stdout, Q3_VERSION " dedicated server (%s)\n", date );
 #else