[RFC]: Detection of OpenBLAS interface bitwidth

This is my strange way of beginning discussion on the best way to incorporate this into the BLAS code.

It would be very useful to; at the least, print a warning when Julia's `USE_LIB64` setting conflicts with OpenBLAS's `INTERFACE64`, or even switch Julia's `USE_LIB64` setting to match.  This second option may not be feasible, as e.g. `arpack` and `suite-sparse`, etc... would need to match, so maybe informing the user is the best course of action here.

Where is a good place to stick this kind of code?  It would be good (in my mind) to print this out when Julia's BLAS facilities are first initialized, but maybe not every time a BLAS function is called. :)