Under OSX add an underscore prefix for the function name being passed
to dlsym(). [1] suggests this shouldn't be necessary but testing seems
to have shown that dlsym() wasn't working under OSX. This is an attempt
to see if this fixes the issue

This has not been tested at all!

[1] https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man3/dlsym.3.html