Add a definition of alloca

Benchmarks not including headers files fail as we only provided a definition of
__builtin_alloca, which alloca expands to in standard libary headers, but none
for alloca directly.

src/ansi-c/library/stdlib.c

@@ -147,6 +147,18 @@ inline void *__builtin_alloca(__CPROVER_size_t alloca_size)
   return res;
 }
 
+/* FUNCTION: alloca */
+
+#undef alloca
+
+void *__builtin_alloca(__CPROVER_size_t alloca_size);
+
+inline void *alloca(__CPROVER_size_t alloca_size)
+{
+__CPROVER_HIDE:;
+  return __builtin_alloca(alloca_size);
+}
+
 /* FUNCTION: free */
 
 #undef free