Update management of linear algebra libraries

Management of detection and/or linking to BLAS and LAPACK libraries is
re-organized. The code has migrated to its own module. This will
help future extension and maintenance. This version communicates
things that are newsworthy and stays out of the way when nothing
is changing.

We no longer over-write the values specified by the user for
GMX_EXTERNAL_(BLAS|LAPACK). Previously, this was used to signal
whether detection succeeded, but that does not really get the job
done. Instead, the user is notified that detection failed (repeatedly,
if they deliberately set such an option on).

Correct usage and expected behaviour in all cases is documented both
in the code and the install guide.

The user interface is pretty much unchanged. We still don't offer full
configurability (e.g. MKL for FFTs must use MKL for linear algebra
unless GMX_*_USER is used, and the only way to get MKL for linear
algebra is to use it for FFTs). The size of any performance difference
is probably very small, and if the user really needs mdrun with
certain FFT and tools with certain linear algebra library, they can do
two configurations. Note that mdrun never calls any linear algebra
routines (tested empirically)!

Expanded the solution of #771 by testing that the user supplied
libraries that actually work. If not, we emit a warning and try to use
them anyway.

We also now check that MKL really does provide linear algebra
routines, and fall back to the default treatment if it does not.

Refs #771,#1186

Change-Id: Ife5c59694e29a3ce73fc55975e26f6c083317d9b