$NetBSD$

Make sure __builtin_alloca is defined before using it

--- src/config.h.in.orig	2021-07-26 11:30:35.000000000 +0000
+++ src/config.h.in
@@ -910,8 +910,10 @@ things are arranged in config.h.in.  In 
 #include <alloca.h>
 #elif defined (__GNUC__)
 #ifndef alloca
+#ifdef __builtin_alloca
 #define alloca __builtin_alloca
 #endif
+#endif
 #elif defined (WIN32_NO_CONFIGURE)
 /* Defines size_t and alloca ().  */
 #include <malloc.h>