allow compile-time suffix for BLAS functions

For using scs in Julia (karanveerm/SCS.jl#1), we'd like to use the high-quality BLAS/LAPACK (openblas) that already ships with Julia. For a number of technical reasons, Julia appends its own suffix to BLAS functions, e.g., ``daxpy_64_``. This change lets us pass in the suffix with ``-DBLASSUFFIX=_64_``. Also, ``-DNOBLASUNDERSCORE`` is replaced by ``-DBLASSUFFIX=``

CC @tkelman
If ``-DBLASSUFFIX=_`` works (standard case), then we should be fine.