Use libmingwex for gmtime_s.

gmtime_s first appeared in MSVCR80, but libmingwex has a helper function
that tries to find the symbol or falls back to an internal
implementation.

(Patch by Kenny Root.)

Change-Id: I96ef9cd7459d7e8202831a4e687dfbc055c9f50b
Reviewed-on: https://boringssl-review.googlesource.com/4091
Reviewed-by: Adam Langley <agl@google.com>
1 file changed