undefined reference to `getpwnam_r@@GLIBC_2.0'

[Krzys Majewski <majewski@cs.ubc.ca>]

I'm getting these errors when compiling stuff:

/usr/X11R6/lib/libX11.so.6: undefined reference to `getpwnam_r@@GLIBC_2.0'
/usr/X11R6/lib/libX11.so.6: undefined reference to `getpwuid_r@@GLIBC_2.0'

I  upgraded my libc  from "stable"  to "unstable"  last night,  so I'm
guessing that's the problem. How  can I downgrade it back to "stable"?
(tried apt-get install --reinstall but no go..) 
-chris