configure: Don't force _WIN32_WINNT to 0x0601 if it is set to a higher version

Since e807c8e9, we're explicitly unconditionally defining _WIN32_WINNT,
regardless if it was set before or not.

If it was set before (e.g. on the command line), to a higher version,
we should honor that.

This is cruicial for WinRT building, where _WIN32_WINNT is set to Windows
8 or higher on the command line.

configure.ac

@@ -202,10 +202,10 @@ case "${host_os}" in
     CXXFLAGS="${CXXFLAGS} -static-libgcc"
     AC_CHECK_TOOL(WINDRES, windres, :)
     AC_CHECK_TOOL(OBJCOPY, objcopy, :)
-    AH_TOP([#if defined(_WIN32) && !defined(_WIN32_WINNT)])
+    AH_TOP([#if defined(_WIN32) && (!defined(_WIN32_WINNT) || _WIN32_WINNT < 0x0601)])
+    AH_TOP([# undef _WIN32_WINNT])
     AH_TOP([# define _WIN32_WINNT 0x0601 /* Windows 7 */])
     AH_TOP([#endif])
-    AC_DEFINE([_WIN32_WINNT], 0x0601, [Define to '0x0601' for Windows 7 APIs.])
     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.])