math/linbox: fix build with clang 19
Clang 19 has become more strict about errors in member functions, which
results in errors building math/linbox:
../../linbox/vector/blas-subvector.h:121:20: error: no member named 'data' in 'BlasSubvector<_Vector>'
121 | _ptr(V.data()+beg), _size(dim), _inc(inc), _field(&V.field()) {}
| ~ ^
../../linbox/matrix/sparsematrix/sparse-ell-matrix.h:1216:16: error: cannot assign to non-static data member '_ld' with const-qualified type 'const size_t &' (aka 'const unsigned long &')
1216 | _ld = iter._ld ;
| ~~~ ^
../../linbox/matrix/sparsematrix/sparse-ellr-matrix.h:1108:12: error: no viable overloaded '='
1108 | _rowid = iter._rowid;
| ~~~~~~ ^ ~~~~~~~~~~~In all these cases, the methods where the errors occur are not actually
used, so until upstream sorts out their incorrect code, stub them out
using #if 0/#endif pairs.
PR: 284159
Approved by: thierry (maintainer)
MFH: 2025Q1