OpenCL build failures now show the build logs even without debug build.

Basically one always wants to see the build errors,
because there is nothing one can do without fixing them.
Requesting a separate debug-build is an unnecessary indirection.
Fixed erroneous is_division<> meta function.

This has lead the OpenCL element-wise division to fail.
Added OpenCL kernels for element-wise operations.

Kernels for x += element_op(y) and x -= element_op(y) are included as well,
even though the expression template parser does not take them into account yet.