Fix Chromium NaCl build.

Chromium's NaCl build has _POSIX_SOURCE already defined, so #undef it first.
The compiler used also dislikes static asserts with the same name.

Change-Id: I0283fbad1a2ccf98cdb0ca2a7965b15441806308
Reviewed-on: https://boringssl-review.googlesource.com/5430
Reviewed-by: Adam Langley <agl@google.com>