configure: Default to _WIN32_WINNT=0x0502 (XP) as minimum, for legacy mingw

This makes the getaddrinfo functions visible, which aren't normally
by default on legacy mingw.

We already force __MSVCRT_VERSION__ to an XP version.
Signed-off-by: Martin Storsjö <martin@martin.st>

configure

@@ -4133,6 +4133,8 @@ probe_libc(){
         add_${pfx}cppflags -U__STRICT_ANSI__ -D__USE_MINGW_ANSI_STDIO=1
         check_${pfx}cpp_condition _mingw.h "__MSVCRT_VERSION__ < 0x0700" &&
             add_${pfx}cppflags -D__MSVCRT_VERSION__=0x0700
+        check_${pfx}cpp_condition windows.h "defined(_WIN32_WINNT) && _WIN32_WINNT < 0x0502" &&
+            add_${pfx}cppflags -D_WIN32_WINNT=0x0502
         eval test \$${pfx_no_}cc_type = "gcc" &&
             add_${pfx}cppflags -D__printf__=__gnu_printf__
     elif check_${pfx}cpp_condition crtversion.h "defined _VC_CRT_MAJOR_VERSION"; then