Allow overriding _WIN32_WINNT with environment variable

configure.ac

@@ -205,7 +205,8 @@ case "${host_os}" in
   *mingw32* | *cygwin* | *wince* | *mingwce*)
     AC_CHECK_TOOL(WINDRES, windres, :)
     AC_CHECK_TOOL(OBJCOPY, objcopy, :)
-    AC_DEFINE([_WIN32_WINNT], 0x0502, [Define to '0x0502' for Windows XP SP2 APIs.])
+    test -z "$_WIN32_WINNT" && _WIN32_WINNT=0x0502
+    AC_DEFINE_UNQUOTED([_WIN32_WINNT], ${_WIN32_WINNT}, [Windows API level])
     AC_DEFINE([_WIN32_IE], 0x0600, [Define to '0x0600' for IE 6.0 (and shell) APIs.])
     AC_DEFINE([_UNICODE], [1], [Define to 1 for Unicode (Wide Chars) APIs.])
     AC_DEFINE([UNICODE], [1], [Define to 1 for Unicode (Wide Chars) APIs.])