MinGW versus _strtoui64() ?

Per a comment from Petr Jelinek, I added this in commit 23a27b039d94ba35:

#ifdef WIN32  return _strtoui64(str, endptr, base);
#else ...

Several of the Windows buildfarm members are good with that, but
narwhal is not:

numutils.c: In function `pg_strtouint64':
numutils.c:406: warning: implicit declaration of function `_strtoui64'
...
Creating library file: libpostgres.a
utils/adt/numutils.o(.text+0x67b): In function `pg_strtouint64':
C:/msys/1.0/local/pgbuildfarm/buildroot/HEAD/pgsql.build/src/backend/utils/adt/numutils.c:406: undefined reference to
`_strtoui64'
collect2: ld returned 1 exit status

Any thoughts how to fix that?
        regards, tom lane