Minimum GCC version for building glibc

[Paul Eggert]

Joseph S. Myers wrote:
> * Should 4.6 or 4.7 be the minimum?

As we've recently run into multiple reasons why requiring 4.7 would be helpful 
(atomics and stdalign) I suggest going with 4.7.  Also, it's less hassle to 
switch to 4.7 now, than to switch to 4.6 now and 4.7 letter.

For what it's worth, 4.4's last release and 4.7's first release were both March 
2012.