pgsql: Rewrite strnlen replacement implementation from 8a241792f96.

Rewrite strnlen replacement implementation from 8a241792f96.

The previous placement of the fallback implementation in libpgcommon
was problematic, because libpqport functions need strnlen
functionality.

Move replacement into libpgport. Provide strnlen() under its posix
name, instead of pg_strnlen(). Fix stupid configure bug, executing the
test only when compiled with threading support.

Author: Andres Freund
Discussion: https://postgr.es/m/E1e1gR2-0005fB-SI@gemulon.postgresql.org

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/fffd651e83ccbd6191a76be6ec7c6b1b27888fde

Modified Files
--------------
configure | 25 ++++++++++++++++++++++++-
configure.in | 6 +++---
src/backend/utils/mmgr/mcxt.c | 3 +--
src/common/string.c | 20 --------------------
src/include/common/string.h | 15 ---------------
src/include/pg_config.h.in | 4 ++++
src/include/pg_config.h.win32 | 10 +++++++---
src/include/port.h | 4 ++++
src/port/snprintf.c | 4 +---
src/port/strnlen.c | 33 +++++++++++++++++++++++++++++++++
10 files changed, 77 insertions(+), 47 deletions(-)