Compilation with openblas fixes

[gkaf89]

These are a few fixes in the compilation with OpenBLAS. The issues rectified are the following:

1. Allow multiple directories in CMAKE_PREFIX_PATH, e.g. -D CMAKE_PREFIX_PATH:PATH="${HOME}/opt/scalapack;${HOME}/opt/openblas"
2. Link with the correct LAPACK library wehn building with OpenBLAS. OpenBLAS links BLAS and LAPACK in a single shared object, libopenblas.so. The reference implementation of Netlib is used, with some parts optimized.
3. Report support for GEMMT by initializing the flag BLAS_HAVE_GEMMT.

[scivision]

Thanks I used two of your commits, but implemented OpenBLAS in a more general way.
ad06d43
5a7fd7e

I also added a CI test for OpenBLAS https://github.com/scivision/mumps/actions/runs/5555844546/jobs/10147547056.

To specify OpenBLAS:

cmake -Bbuild -DLAPACK_VENDOR=OpenBLAS

# optional if having trouble auto-finding OpenBLAS
cmake -Bbuild -DLAPACK_VENDOR=OpenBLAS -DLAPACK_ROOT=/path/to/openblas

[gkaf89]

Thank you for the quick merge!

Don't worry about modifying my approach, I am new in the code base and thus still a bit reluctant to make extended refactorings.