Use __builtin_alloca if no other option.
#define alloca _alloca
#elif defined(JANET_LINUX)
#include <alloca.h>
+#elif !defined(alloca)
+/* Last ditch effort to get alloca - works for gcc and clang */
+#define alloca __builtin_alloca
#endif
#define JANET_FFI_MAX_RECUR 64
