Filter -fno-delete-null-pointer-checks for clang

Fixes #27836.
qrupdate, slicot: improved BLAS/LAPACK support

qrupdate.rb: added OpenBLAS option. Compiled with -ff2c flag for
    -framework vecLib to ensure compatibility.
slicot.rb: removed all mention of BLAS/LAPACK, because only the static
    library is built.

Closes #27773.

Signed-off-by: Misty De Meo <mistydemeo@gmail.com>