Update xetex to 0.9997.5.

Changes:
        * xetex.ch (pack_buffered_name): Adapt to modified ../tex.ch.
        (read_font_info): Cast print_c_string() arg to string.
        * xetex.ch: Reformulate to not depend on eTeX_version_string.
        * XeTeXFontInst.h: Fix prototype for xmalloc().
        * XeTeXFontMgr_FC.cpp, XeTeX_ext.c: Move (nested) extern
        declaration of gFreeTypeLibrary from here ...
        * XeTeX_ext.h: ... to here.
        * XeTeXOTLayoutEngine.{cpp,h}: Drop support for ICU < 4.2.

(minskim)