drop alloca #define

(Because GNU's alloca.h unconditionally #undef's alloca)

Also, remove gcc specific sections in headers. and
instead change tests such that gcc does not use them.

win32/include/stddef.h

@@ -18,9 +18,6 @@ typedef int int32_t;
 typedef long long int int64_t;
 #endif
 
-#ifdef __i386__
-void *_alloca(size_t);
-#define alloca _alloca
-#endif
+void *alloca(size_t);
 
 #endif