We can remove those C++11 and C++14 kludges because we require GCC 4.9
now.

The "pure" and "const" attributes are not so well-defined, and a
recent clang version implements an optimization which pushes the
definition's boundary beyond what I believed it was.  clang now
assumes that functions declared "pure" cannot throw exceptions, even
if they lack the "noexcept" specification.

When compiled with this new clang version, MPD will crash randomly if
an exception happens to get thrown by such as "pure" function
(https://github.com/MusicPlayerDaemon/MPD/issues/41).

This commit removes all such misplaced "pure" and "const" attributes,
closing #41.

This eliminates some overhead, because the compiler doesn't need to
consider these functions throwing.

Just in case.

GCC7 outputs the following error without this change:

    src/util/ReusableArray.hxx:61:35: error: no matching function for call to ‘swap(size_t&, const size_t&)’
       std::swap(capacity, src.capacity);

which can be resolved by just using an rvalue-reference rather than a
const rvalue-reference.
Signed-off-by: Ben Boeckel <mathstuf@gmail.com>

Move code from SongFilter.cxx.