Support calling custom functions from lambda expressions

[kylelutz]

It should be possible to call custom functions from lambda expressions.

For example, this should be possible:

BOOST_COMPUTE_FUNCTION(float, square, (float x),
{
    return x * x;
});

transform(vec.begin(), vec.end(), vec.begin(), square(2 * _1), queue);

[kylelutz]

This has been requested multiple times during the Boost.Compute review. Bumping it up in priority.