Fix --with-winhttp configure option

It was never defined because USE_WIN32 wasn't set at the time it was
tested. Fix this by just always defining this option, even though it
only makes sense for MSW.

Also fix the test for winhttp.h, it needs to include windows.h first, as
otherwise compiling the header would always fail.

configure

@@ -5379,7 +5379,6 @@ fi
 
           eval "$wx_cv_use_libcurl"
 
-if test "$USE_WIN32" = 1; then
 
           withstring=
           defaultval=$wxUSE_ALL_FEATURES
@@ -5409,7 +5408,6 @@ fi
 
           eval "$wx_cv_use_winhttp"
 
-fi
 if test "$USE_DARWIN" = 1; then
 
           withstring=
@@ -23136,11 +23134,12 @@ fi
         LIBS="$LIBS -loleacc"
     fi
     if test "$wxUSE_WINHTTP" = "yes" ; then
-        ac_fn_c_check_header_mongrel "$LINENO" "winhttp.h" "ac_cv_header_winhttp_h" "$ac_includes_default"
+        ac_fn_c_check_header_compile "$LINENO" "winhttp.h" "ac_cv_header_winhttp_h" "#include <windows.h>
+"
 if test "x$ac_cv_header_winhttp_h" = xyes; then :
 
 else
-   wxUSE_WINHTTP=no
+  wxUSE_WINHTTP=no
 fi