Require GCC 4.4 or later to build glibc

[Roland McGrath]

> This patch makes configure require GCC 4.4 or later and updates the
> documented version requirement.

OK.

> This patch doesn't change anything about the version *recommendation*,
> but in my view we should just refer to the current stable GCC release
> rather than naming a specific version.

Agreed.  Each individual release can say somewhere that goes into the
tarball what the latest version it has been built and tested with at the
time of release is.