no need to include alloca.h if gcc is used

src/ua_util.h

@@ -31,8 +31,6 @@
 
 #ifdef _WIN32
 # include <malloc.h>
-#else
-# include <alloca.h>
 #endif
 
 #define UA_NULL ((void *)0)
@@ -52,9 +50,14 @@
 #define UA_memset(ptr, value, size) memset(ptr, value, size)
 
 #ifdef _WIN32
-# define UA_alloca(SIZE) _alloca(SIZE)
+    # define UA_alloca(SIZE) _alloca(SIZE)
 #else
-# define UA_alloca(SIZE) alloca(SIZE)
+ #ifdef __GNUC__
+    # define UA_alloca(size)   __builtin_alloca (size)
+ #else
+    # include <alloca.h>
+    # define UA_alloca(SIZE) alloca(SIZE)
+ #endif
 #endif
 
 /********************/