Fixes this warning:

/builddir/build/BUILD/polymake-4.5/include/core/polymake/internal/AVL.h:52:6: warning: type ‘pm::AVL::Ptr_flags’ violates the C++ One Definition Rule [-Wodr]
   52 | enum Ptr_flags : size_t {
      |      ^
/builddir/build/BUILD/polymake-4.5/include/core/polymake/internal/AVL.h:52:6: note: an enum with different value name is defined in another translation unit
   52 | enum Ptr_flags : size_t {
      |      ^
/builddir/build/BUILD/polymake-4.5/include/core/polymake/internal/AVL.h:53:4: note: name ‘NONE’ differs from name ‘END_RING’ defined in another translation unit
   53 |    NONE=0,
      |    ^
/builddir/build/BUILD/polymake-4.5/include/core/polymake/internal/AVL.h:53:4: note: mismatching definition
   53 |    NONE=0,
      |    ^

--- lib/core/include/internal/AVL.h.orig	2021-09-29 06:13:35.000000000 -0600
+++ lib/core/include/internal/AVL.h	2021-10-04 06:41:35.414976853 -0600
@@ -33,6 +33,9 @@
 #include <algorithm>
 #include <cassert>
 
+// Avoid name clash with Singular
+#undef NONE
+
 namespace pm {
 
 /// special tags for find_nearest denoting the first and last occurrence of a given key in a multi-set