cleanup warnings

In particular `_STRINGIFY' is defined in mingw-w64
internal headers. The underscore-prefixed names are
reserved.

v2:

- Align macro definition
- Use q_unused attribute

Reported by: @skullernet

inc/common/common.h

@@ -50,8 +50,8 @@ with this program; if not, write to the Free Software Foundation, Inc.,
 
 #define CONST_STR_LEN(x) x, x ? sizeof(x) - 1 : 0
 
-#define __STRINGIFY(x)  #x
-#define STRINGIFY(x)    __STRINGIFY(x)
+#define STRINGIFY2(x)   #x
+#define STRINGIFY(x)    STRINGIFY2(x)
 
 typedef struct {
     const char *name;

src/windows/system.c

@@ -788,7 +788,7 @@ void Sys_Init(void)
     HMODULE module;
     BOOL (WINAPI * pSetProcessDEPPolicy)(DWORD);
 #endif
-    cvar_t *var = NULL;
+    cvar_t *var q_unused;
 
     // check windows version
 	vinfo.dwOSVersionInfoSize = sizeof(vinfo);