[tlbuild] xetex dependencies

[Karl Berry]

Hi Ken,

    Indeed, texk/web2c/xetexdir/XeTeXFontInst.h includes ft2build.h.

freetype (and ICU and harfbuzz and all that other craziness) is included
in TeX Live, and when I build xetex, the TL-built .a is linked
against. That was the case for Apostolos too. Because, indeed, we do not
build with system libraries preferred.

Best,
Karl