How to refer to the current directory (.)?

[Peng Yu]

I have the following makefile, by which I want it to touch the current
directory if the file 'a' is changed. It seems to be working. But I
want to make sure if '.' has some special meaning in make (by simply
looking though the index, I don't find any special meaning). Could
somebody let me know?

$ cat Makefile
.PHONY: all

all: .

.: a
        touch $@