CMake blas_int

[mgates3]

1. Updates CMake so it always goes through the BLAS++ BLASFinder to detect the Fortran mangling and whether the BLAS is int32 or int64.

2. Updates the GitHub CT so that it checks additional BLAS libraries, adding OpenBLAS, specifying BLAS using CMake's FindBLAS (to verify 1), and specifying BLAS using BLAS_LIBRARIES. BLIS can be added once it is working on ICL's machines.

Details

There are 3 ways to specify the BLAS library:

1. BLAS++ BLASFinder, using blas, blas_int, blas_threaded, etc.:
   cmake -Dblas=openblas ..
2. BLAS_LIBRARIES, which also uses BLASFinder:
   cmake -DBLAS_LIBRARIES='-lopenblas' ..
3. CMake FindBLAS:
   cmake -DBLA_VENDOR=OpenBLAS ..

For blas=xyz, blas_int is a constraint on what libraries are checked. E.g., blas=mkl blas_int=int32 will check only mkl_gf_lp64 and mkl_intel_lp64, not mkl_gf_ilp64 and mkl_intel_ilp64. However, for all libraries, BLAS++ checks both 32-bit and 64-bit int. That is, we detect the int size even though in some cases like MKL, we already know the answer. (Except when cross-compiling.)

For BLAS_LIBRARIES, as above, BLASFinder tests 32-bit int, and if that fails, tests 64-bit int. This is the safest action. Only when cross-compiling do we need to trust that blas_int was correctly set.

For CMake's FindBLAS using BLA_VENDOR, blas_int was never referenced, so BLAS++ would implicitly assume 32-bit integers. With this PR, after FindBLAS, the BLAS_LIBRARIES are run through the BLAS++ BLASFinder to detect int sizes and Fortran mangling.

[mgates3]

Reviewed.