app-eselect/eselect-{blas,lapack}: new design for BLAS/LAPACK switch.

app-eselect/eselect-{blas,lapack}: new design for BLAS/LAPACK switch.

  As discussed in the gentoo-dev, it is decided to adopt the runtime
  switching of BLAS/CBLAS that has been proved by the experience of
  Debian.  The Gentoo libgcc-style ld.so.conf runtime switching has
  been adopted as well, so as to avoid changing symlinks in /usr
  ignoring package manager.

  This design solves all the remaining problems regarding BLAS/LAPACK,
  thus making alternative-2.eclass (runtime switching of build
  options) and proposed USE_EXPAND (PYTHON_TARGET like eclass)
  obsolete.

  Finally we are at it.  Thanks to all for years of ideas, debates and
  hard work.

Reference: https://archives.gentoo.org/gentoo-dev/message/0177fea10e3ee06d85713101d17fc8d6

Closes: https://bugs.gentoo.org/632624
Closes: https://bugs.gentoo.org/470908
Closes: https://bugs.gentoo.org/267691
Closes: https://github.com/gentoo/gentoo/pull/12316
Closes: https://github.com/gentoo/gentoo/pull/12318
Closes: https://github.com/gentoo/sci/pull/835
Closes: https://github.com/gentoo/sci/pull/837

Credit: Dominik Schmidt, TheChymera, kiwifb, lkraav, Alessandro-Barbieri
Signed-off-by: Mo Zhou <cdluminate@gmail.com>
Signed-off-by: Benda Xu <heroxbd@gentoo.org>