io_buffer.c: use mremap based resizing only when mremap available

some libc implementations (e.g. wasi-libc) define MREMAP_MAYMOVE, but
don't have mremap itself, so guard the use of mremap by HAVE_MREMAP

configure.ac

@@ -2032,6 +2032,7 @@ AC_CHECK_FUNCS(mkfifo)
 AC_CHECK_FUNCS(mknod)
 AC_CHECK_FUNCS(mktime)
 AC_CHECK_FUNCS(mmap)
+AC_CHECK_FUNCS(mremap)
 AC_CHECK_FUNCS(openat)
 AC_CHECK_FUNCS(pipe2)
 AC_CHECK_FUNCS(poll)

io_buffer.c

@@ -1239,7 +1239,7 @@ rb_io_buffer_resize(VALUE self, size_t size)
         rb_raise(rb_eIOBufferAccessError, "Cannot resize external buffer!");
     }
 
-#ifdef MREMAP_MAYMOVE
+#if defined(HAVE_MREMAP) && defined(MREMAP_MAYMOVE)
     if (data->flags & RB_IO_BUFFER_MAPPED) {
         void *base = mremap(data->base, data->size, size, MREMAP_MAYMOVE);