make_makefiles: Allow running from a separate worktree.

tools/make_makefiles

@@ -501,7 +501,7 @@ sub update_wine_inf()
 }
 
 my $git_dir = $ENV{GIT_DIR} || ".git";
-die "needs to be run from a git checkout" unless -d $git_dir;
+die "needs to be run from a git checkout" unless -e $git_dir;
 
 my @all_files = split /\0/, `git ls-files -c -z`;
 map { $ignored_source_files{$_} = 1; } split /\0/, `git ls-files -d -z`;