What does "..=../" do?

[Steve Kenton]

I was digging through the glibc make file to see how to rebuild just the 
crtx files and I ran across a make construct I did not understand on the 
following line

make  subdir=csu -C csu ..=../ others

It's building the target others after changing to the subdirectory csu 
and setting the value of the variable subdir to "csu",
but what does the "..=../" do?

I have been unable to find any reference to a special variable ".."

Is is some sort of path manipulation? Any pointers would be appreciated.