mmap-anon: avoid out-of-order autoconf expansion * m4/mmap-anon.m4 (gl_FUNC_MMAP_ANON): Use correct SYSTEM_EXTENSIONS macro to silence warnings from autoconf 2.63b. * modules/memchr-tests (Depends-on): Add extensions. * modules/memchr2-tests (Depends-on): Add extensions. * modules/memcmp-tests (Depends-on): Add extensions. * modules/memmem-tests (Depends-on): Add extensions. * modules/memrchr-tests (Depends-on): Add extensions. Signed-off-by: Eric Blake <ebb9@byu.net>