sci-mathematics/giac changes

The configurescript doesn't find the fltk-libraries and -includes if they're not in /usr/include or /usr/lib/. 
Gentoo unfortunately places them in /usr/include/fltk-1.0 and /usr/lib{32,64}/fltk-1.0.

I do not know if the invocation of an external utility like "fltk-config" is the right way to solve this, since, as noted in the commit, it breaks crosscompilation. If you know a better solution I'll gladly implement it.
user_enable?? What is this?