sys_times, sys_utsname: use include_next

sys_times, sys_utsname: use include_next

Another step towards being able to make GL_LINK_WARNING useful.

* m4/sys_times_h.m4 (gl_SYS_TIMES_H): Support wrapping an existing
header.
(gl_SYS_TIMES_H_DEFAULTS): Add another variable.
* m4/sys_utsname_h.m4 (gl_SYS_UTSNAME_H)
(gl_SYS_UTSNAME_H_DEFAULTS): Likewise.
* modules/sys_times (Depends-on): Add include_next.
(Makefile.am): Substitute additional values.
* modules/sys_utsname (Depends-on, Makefile.am): Likewise.
* lib/sys_times.in.h (includes): Include native header, if
available.
* lib/sys_utsname.in.h (includes): Likewise.
* tests/test-sys_times.c (main): Enhance test.

Signed-off-by: Eric Blake <ebb9@byu.net>