[Dune] [#748] improve configure check for BLAS

[Dune]

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The following task has a new comment added:

FS#748 - improve configure check for BLAS
User who did this - Jö Fahlke (joe)

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Actually, the istl's summary line about blas is probably a bad idea, because
it just reports what was found by ACX_BLAS -- but that is not neccessarily the
version of blas that will actually be used.  SuperLU for instance may provide
it's own version of blas, which is not detected by ACX_BLAS, but the ACX_BLAS
check will be run regardless.

Also the statement that the blas flags don't end up in ALL_PKG_* is only
partially true:  istl does not use blas directly, it is only used via SuperLU,
SuperLU-dist or pardiso.  If DUNE_SUPERLU finds SuperLU, it will add the
neccessary flags to ALL_PKG_*, and that includes the blas flags.

Your patch is probably OK for now.  It is problematic in cases where SuperLU
actually uses its own version of blas, because in that case both the flags of
the system's blas and the flags of SuperLU's internal blas will end up in
ALL_PKG_*.  However, the problem can be avoided by configuring with
--without-blas, and usually SuperLU should use the system's blas anyway.

I would not modify ACX_BLAS, unless we want to push the changes upstream.
Instead I would ditch ACX_BLAS altogether and do all the stuff in DUNE_BLAS
itself.

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