Support functions which call other user-defined functions

[kylelutz]

It should be possible for custom functions (e.g. those created with BOOST_COMPUTE_FUNCTION()) to call other user-defined functions.

[rosenrodt]

I am also interested in having this feature too. Also it would be very helpful if one could create a host- & device-callable function via something like BOOST_COMPUTE_HOST_DEVICE_FUNCTION() that expands to regular C host function as well as compute::function.

Then the meta_kernel class could somehow prepend the stringized device function based on information given by the user (via an extra parameter to BOOST_COMPUTE_FUNCTION()) or done automatically just by searching for matching function name