x11-toolkits/fltk: the port had been updated to version 1.3.8

FLTK 1.3.8 is a maintenance release with minor enhancements. One new
function `fl_choice_n()' has been introduced (please see documentation
for details).

This release does not contain new ABI features and is 100% compatible
with releases 1.3.5-1.3.7.