pkg-config free detection of installed MKL

[termoshtt]

As https://github.com/rust-math/intel-mkl-src/pull/36#issuecomment-643576170, pkg-config setting of official apt/yum repository is broken.

TODO

• [ ] Create test container for apt/yum installation like #36
• [ ] Seek mkl using environment variable e.g. MKLROOT which specify where the MKL is installed

[termoshtt]

Related to never-download option discussed in #29