Fix ExternalBLASLAPACK.cmake to handle BLA_VENDOR

[hughcars]

Right now the build for BLAS and LAPACK does not respect BLA_VENDOR. If BLA_VENDOR=OpenBLAS is set for instance, but armpl, mkl or aocl are found, it will override. This issue is to remove this limitation. This will require refactoring to do neatly, and should take advantage of the addition of AOCL to the handled list of vendors in findBLAS.cmake.