minor changes to get rid of gcc warnings, especially on 64-bit system.

minor changes to get rid of gcc warnings, especially on 64-bit system.

* libxslt/extensions.c, libxslt/functions.c, libxslt/numbers.c,
  libxslt/pattern.c, libxslt/transform.c, libxslt/xslt.c,
  libxslt/xsltInternals.h: minor changes to get rid of gcc
  warnings, especially on 64-bit system. Implemented
  XML_CAST_FPTR macro to cast between function pointer <->
  object pointer while avoiding gcc warnings (a hack).
  No change to the logic.