Fix warnings

[Kerilk]

This fixes several classes of warnings that are creating a lot of noise in the build. Those may be hiding issues we would like to fix. I will rebase on #218 as I didn't want to muddy the waters by adding those commit to the existing PR. The classes of issues fixed are:

• Explicitly cast unused arguments to (void);
• Cast cl_uint used as loop bounds to (int) to compare against the loop counter;
• Initialize an array to 0;
• Remove unused variable;
• Remove extra ; when unneeded after macros;
• Add guards around static functions so that they are defined only when used.

Once this and #218 are merged, remaining warnings need to be investigated, and we may think about activating -Werror at least on Linux and MacOS targets.

[bashbaug]

Thanks, LGTM! Still a few more but this is a nice improvement...