[c-host] dont use DX12 feature levels on Windows8

Fixes #218

common/include/common/windebug.h

@@ -21,6 +21,7 @@ Place, Suite 330, Boston, MA 02111-1307 USA
 
 #include "debug.h"
 #include <windows.h>
+#include <stdbool.h>
 
 #ifdef __cplusplus
 extern "C" {
@@ -30,6 +31,8 @@ void DebugWinError(const char * file, const unsigned int line, const char * func
 
 #define DEBUG_WINERROR(x, y) DebugWinError(STRIPPATH(__FILE__), __LINE__, __FUNCTION__, x, y)
 
+bool IsWindows8();
+
 #ifdef __cplusplus
 }
 #endif