backed out the last change and re-did it the "right way" (bug 158372).

* libxslt/variables.c, libxslt/xsltInternals.h: backed out the
  last change and re-did it the "right way" (bug 158372).
* tests/general/bug-158.*, tests/general/Makefile.am,
  tests/docs/bug-158.*, tests/general/Makefile.am: added test
  case for this bug