gettimeofday: expose type of second argument

gettimeofday: expose type of second argument

Needed to silence compiler warnings such as:
test-gettimeofday.c:23: warning: initialization from incompatible pointer type
based on whether gettimeofday complies with POSIX or provides
the extension of struct timezone.

* m4/gettimeofday.m4 (gl_FUNC_GETTIMEOFDAY): Do better detection
of glibc extension signature, and define GETTIMEOFDAY_TIMEZONE.
* tests/test-gettimeofday.c: Use it to silence warning.
* doc/posix-functions/gettimeofday.texi (gettimeofday): Document
the issue.

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