fix a few CLang warnings

most of them quite insignificant, but still annoying. The only
exception is the warning about the changed alignment for the 'work'
argument of spu_implicit_assemble(). AFAICT, the only reason why it
worked was that the pointer produced by malloc() was passed
directly. (malloc() seems to fulfill all alignment criteria.) To fix
this, I've changed that argument's type from char* to double*.
better fix for the alignment warning of the previous patch

'work' is a small, constant size array for temporary data so it can be
allocated on the stack and the 'work' parameter can be eliminated entirely.

Thanks to Bård Skaflestad for the suggestion!
fix new 'unused variable' warnings on GCC

that one was due to the fact that the constructor arguments were no
longer used to initialize (unused) private member variables. These
warnings did not appear in CLang for some reason. Again, thanks to
Bård Skaflestad for the review.
Merge pull request #288 from andlaus/fix-clang-warnings

fix a few CLang warnings