Cygwin FAQ has the following info for 'Where is glibc?' : Cygwin does not provide glibc. It uses newlib instead, which provides much (but not all) of the same functionality. Porting glibc to Cygwin would be difficult.
I was surprised and checked out the release packages as i had earlier used it. While i checked the repositories, it appears that glibc was actually indeed part of cygwin until version 2.10. Can anyone tell of the porting difficulty for the subsequent versions of glibc ?