util: initialize locale_t with a static object

util: initialize locale_t with a static object

_mesa_strtod and _mesa_strtof may be called from multiple threads.  They need
to be thread-safe.

v2: platform checks are now done in configure.ac

Signed-off-by: Chia-I Wu <olv@lunarg.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>