* m4/strerror_r.m4 (gl_FUNC_STRERROR_R): Move requirement of
gl_USE_SYSTEM_EXTENSIONS from here ...
(gl_FUNC_STRERROR_R_WORKS): ... to here, which is the macro that
actually needs it. This avoids a bug where 'configure' tests
whether strerror_r has the POSIX API before enabling GNU
extensions. On Android, enabling GNU extensions switches from the
POSIX to the GNU API. Problem reported by Lucy Phipps in:
https://lists.gnu.org/r/bug-gnulib/2021-09/msg00026.html