The patch in the bug report isn't quite right (aka remove the header and guards all together).

Instead one should add a configure check - AC_LINK_IFELSE() ... which:
 - includes xlocale.h reusing the HAVE_XLOCALE_H guard
 - references strtod_l w/o any guard, setting a macro HAVE_FOO
Then update the code
 - swap the __GNU_SOURCE guards through the file with HAVE_FOO
 - HAVE_XLOCALE_H should guard _only_ the header inclusion

Created attachment 133909 [details] [review]
WIP patch

Work in progress patch

The locale_t check does not seem like a good idea.

The struct is used by {new,free}locale API, which pulls locale.h.
Aka the struct may be available, even when strto{f,d}_l is not.

(In reply to Laurent carlier from comment #2)
> Created attachment 133909 [details] [review] [review]
> WIP patch
> 
> Work in progress patch

Your patch checks for `locale_t`, but I don't think this is the thing that might be missing; `strtod_l()` is.
I had also started working on this, I just pushed my WIP here:
https://github.com/1ace/mesa  wip/strtod-fix

(quick diff link:)
https://github.com/mesa3d/mesa/compare/master...1ace:wip/strtod-fix

I'm having a couple issues though, I'm on IRC if you want to work together :)

Fixed by:

commit 49b428470e28ae6ab22083e43fa41abf622f3b0d
Author: Eric Engestrom <eric.engestrom@imgtec.com>
Date:   Thu Aug 31 16:55:56 2017 +0000

    util: improve compiler guard
    
    Glibc 2.26 has dropped xlocale.h, but the functions needed (strtod_l()
    and strdof_l()) can be found in stdlib.h.
    Improve the detection method to allow newer builds to still make use of
    the locale-setting.