Fix building on MinGW: default WINVER is too old (#1681)

MinGW defaults `WINVER` to something very old,
while benchmark requires features gated by `WINVER = 0x0600`,
so manually set update to that.

src/sysinfo.cc

@@ -15,6 +15,10 @@
 #include "internal_macros.h"
 
 #ifdef BENCHMARK_OS_WINDOWS
+#if !defined(WINVER) || WINVER < 0x0600
+#undef WINVER
+#define WINVER 0x0600
+#endif // WINVER handling
 #include <shlwapi.h>
 #undef StrCat  // Don't let StrCat in string_util.h be renamed to lstrcatA
 #include <versionhelpers.h>