Minor enhancements to eliminate compile/test warnings

Minor enhancements to eliminate compile/test warnings
* libxslt/xsltconfig.h.in, libexstl/exsltconfig.h.in:
  changed macro ATTRIBUTE_UNUSED for gcc so that, if undefined,
  it's defined as __attribute__((unused))
* python/libxslt.c: fixed ATTRIBUTE_UNUSED to appear after
  variable declaration
* libxslt/preproc.c: minor change to get rid of unused var/code