Make sure product_version is set for OS X.

This may go away when we move to cmake for good.

Xcode/scripts/increment_version.pl

@@ -14,6 +14,7 @@ print FH <<"EOF";
 extern const unsigned long version_num = $ver;
 extern const char *const version_date = "$date";
 extern const char *const version_time = "$time";
+extern char const * const product_version = "5.0-git-UNKNOWN";
 EOF
 close FH;
 move '.ver.tmp', 'ver.cpp';