fix auto exclude in-work-tree build dirs

updates: -- configure: add build directory to gitignore #2543 -- i3/i3#2543

configure.ac

@@ -154,8 +154,9 @@ else
 	print_BUILD_MANS=no
 fi
 
-git_dir=`git rev-parse --git-dir 2>/dev/null`
-if test -n "$git_dir"; then
+in_git_worktree=`git rev-parse --is-inside-work-tree 2>/dev/null`
+if "$in_git_worktree" == "true"; then
+	git_dir=`git rev-parse --git-dir 2>/dev/null`
 	srcdir=`dirname "$git_dir"`
 	exclude_dir=`pwd | sed "s,^$srcdir,,g"`
 	if ! grep -q "^$exclude_dir" "$git_dir/info/exclude"; then