Blind attempt to fix _configthreadlocale() failures on MinGW.
Apparently, some builds of MinGW contain a version of
_configthreadlocale() that always returns -1, indicating failure.
Rather than treating that as a curl-up-and-die condition, soldier on
as though the function didn't exist. This leaves us without thread
safety on such MinGW versions, but we didn't have it anyway.
Discussion: https://postgr.es/m/
d06a16bc-52d6-9f0d-2379-
21242d7dbe81@2ndQuadrant.com