merge r15207=879 from trunk:

Allow control over the setting of _WINNT_WINNT

Default minimum Windows version:
  0x0600 (Vista/Windows Server 2008)


git-svn-id: https://svn.apache.org/repos/asf/apr/apr/branches/1.4.x@1520880 13f79535-47bb-0310-9956-ffa450edef68