Avoid boost functions that don't work on BSDs.

Avoid boost functions that don't work on BSDs.

As was noted in issue #2261, the function boost::math::iround cannot be
used on some BSD variants due to the following compilation error:

In file included from /root/workspace/dealii/source/lac/lapack_full_matrix.cc:25:
In file included from /usr/local/include/boost/math/special_functions/round.hpp:15:
In file included from /usr/local/include/boost/math/special_functions/fpclassify.hpp:19:
In file included from /usr/local/include/boost/math/special_functions/math_fwd.hpp:26:
In file included from /usr/local/include/boost/math/special_functions/detail/round_fwd.hpp:12:
/usr/local/include/boost/math/tools/promotion.hpp:141:10: error: static_assert failed
"Sorry, but this platform does not have sufficient long double support for the special functions to be reliably implemented."
         BOOST_STATIC_ASSERT_MSG((0 == ::boost::is_same<type, long
         double>::value),
         "Sorry, but this platform does not have sufficient long double support for the special functions to be reliably implement ed.");
/usr/local/include/boost/static_assert.hpp:31:45: note: expanded from macro 'BOOST_STATIC_ASSERT_MSG'

Since we only use iround for clarity this is not hard to work around.

A note on the implementation: LAPACK functions can usually be run in two
different modes. In the first, they compute the optimal size of the work
array. In the second they actually execute the function. Therefore
all we need to do is make the work arrays one longer and we should still
get the same performance out of LAPACK without needing to worry about
any unforeseen roundoff issues.