Adding a test program to check thread reentrancy

* xsltproc/testThreads.c: based loosely on libxml2 one, checks
  concurrent use of the same stylesheet and extensions reentrancy
* config.h.in configure.in: we need to check for pthreads
* Makefile.am xsltproc/Makefile.am: add the new program and insert
  in make check