(ITS#8269) win32: avoid warning about using strdup instead of _strdup

[nacho.resa@gmail.com]

Full_Name: Ignacio Casal Quinteiro
Version: 
OS: Windows 7
URL: ftp://ftp.openldap.org/incoming/
Submission from: (NULL) (2a01:111f:9fa:7f00:864b:f5ff:fe34:ddaa)


You can find the patch here:
https://github.com/nice-software/lmdb/commit/67a61ed18ddb978f4e58198ca55b979039e51ecc