Require GCC 5 or later to build glibc (bug 23993)

[Joseph Myers]

On Tue, 18 Dec 2018, Adhemerval Zanella wrote:

> Wouldn't be better to focus on change toolchain requirements at the start
> of release cycle?

This particular patch is recognizing a requirement that has already been 
introduced (two months ago - it evidently took a long time for it to be 
noticed) as a side effect of using atomic_load_relaxed on a pointer to 
const (I think - in turn, via how the default atomic_load_relaxed 
constructs an asm output using typeof, resulting in that asm output 
variable being const and the error quoted in the bug report).  Given that 
we have such a requirement in fact on some architectures, and given how 
long we've had 4.9 as official minimum GCC version for building glibc, it 
seemed to make more sense to make the requirement for GCC 5 an official 
one everywhere rather than trying to figure out how to get things working 
on older compilers.

-- 
Joseph S. Myers
joseph@codesourcery.com