Function to call to make a killed region available to other programs.
Most window systems provide a facility for cutting and pasting
text between different programs, such as the clipboard on X and
MS-Windows, or the pasteboard on Nextstep/Mac OS.
This variable holds a function that Emacs calls whenever text is
put in the kill ring, to make the new kill available to other
programs. The function takes one argument, TEXT, which is a
string containing the text which should be made available.