Ensure mktime sets errno on error (bug 23789)

Ensure mktime sets errno on error (bug 23789)

Posix mandates that mktime set errno to EOVERFLOW
on error, that it, upon retuning -1, but the glibc
mktime does not so far on 32-bit architectures.

Fix this and add a test to prevent regressions.

The test was run through 'make check' on i686-linux-gnu,
then the fix was added and 'make check' run again.

	* time/mktime.c
	(mktime): Set errno to EOVERFLOW on error.
	* time/bug-mktime4.c: New file.
        * time/Makefile: Add bug-mktime4.