Which Linux distributions are the developers of glibc using?

[james harvey]

I started with Redhat and moved to Fedora when it came out, and have
stuck with it.

Trying to upgrade my glibc to the git master has, for the first time,
exposed me to distribution-specific patches.

There can be quite a few conflicts trying to merge in the Fedora
patches for their latest release of a package, and that project's
current source tree.

I could be wrong, of course, but I assume the glibc developers are
using a distribution that allows them to just install the glibc
source, without distribution-specific patches on top to complicate
their development.  Which ones are these?  I figure Linux From Scratch
would qualify for what I'm looking for, but want to know my options.