mkl: update checks [use sequential, add -lm, skip bad-paths]
- use mkl_sequential [and not the threaded versions]
- skip non-existant guess dir combinations
- add in -lm - as some configures fail if relying on -lm at the very end of compile
- work with --with-blas-lapack-dir=[mkl,mkl/lib,mkl/lib/intel64]