bigint-syrk-blas: Unit tests fail if FLINT library compiled without BLAS

[vasdommes]

In unit_tests for #142, we are using fmpz_mat_mul_blas as one of the matrix multiplication methods (mathematically equivalent to our new matrix multiplication algorithm).

If FLINT is built without BLAS, then the condition #if FLINT_USES_BLAS && FLINT_BITS == 64 does not hold, and fmpz_mat_mul_blas simply returns 0.

We do not use fmpz_mat_mul_blas in production code, only in tests:

sdpb/test/src/unit_tests/cases/calculate_matrix_square.test.cxx

Line 70 in 357a86b

fmpz_mat_mul_blas(Q.fmpz_matrix, PT_bigint.fmpz_matrix,

Thus we don't actually need to have FLINT with BLAS. We should fallback e.g. to fmpz_mat_mul_multi_mod (it also uses CRT), if fmpz_mat_mul_blas doesn't work.

P.S. We also do not check the return code of fmpz_mat_mul_blas, so the error message is not obvious (in fact, it means that Q_result_fmpz_mat_mul_blas is filled with zeros):

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
unit_tests is a Catch2 v3.4.0 host application.
Run with -? for options

-------------------------------------------------------------------------------
calculate_Block_Matrix_square
  P_width=1
  P_height=1 num_blocks=1
  use_dist_blocks=0
-------------------------------------------------------------------------------
../test/src/unit_tests/cases/calculate_matrix_square.test.cxx:117
...............................................................................

../test/src/test_util/diff.hxx:73: FAILED:
  REQUIRE( Abs(a - b) < eps * (Abs(a) + Abs(b)) )
with expansion:
  0.37514 < 9.52083e-117
with messages:
  input: dense tall NxK matrix P, splitted horizontally into blocks
  output: NxN matrix Q := P^T P
  We calculate Q with different methods, including our bigint_syrk_blas,, and
  compare the results.
  El::mpi::Rank() := 0
  El::mpi::Rank(comm) := 0
  bits = El::gmp::Precision() := 768 (0x300)
  diff_precision = bits / 2 := 384 (0x180)
  Initialize P_matrix and calculate local Q with builtin Elemental and FLINT
  methods
  DIFF(Q_result_El_Syrk, Q_result_fmpz_mat_mul_blas)
  diff El::Matrix
  row := 0
  col := 0
  DIFF(a.Get(row, col), b.Get(row, col))
  a := 0.37514
  b := 0
  a - b := 0.37514
  diff_precision := 384 (0x180)
  eps := 2.53794e-116