unix/mpconfigport.h: For MICROPY_NO_ALLOCA=1, don't even include alloca.h.

unix/mpconfigport.h

@@ -232,8 +232,11 @@ extern const struct _mp_obj_fun_builtin_t mp_builtin_open_obj;
     void *mmap_region_head; \
 
 // We need to provide a declaration/definition of alloca()
+// unless support for it is disabled.
+#if !defined(MICROPY_NO_ALLOCA) || MICROPY_NO_ALLOCA == 0
 #ifdef __FreeBSD__
 #include <stdlib.h>
 #else
 #include <alloca.h>
 #endif
+#endif