On UNIX, run configure with the --enable-xslt --with-xslt-sablot options.
Make sure you have the same libraries linked to the Sablotron library as those, which are linked with PHP. The configuration options: --with-expat-dir=DIR --with-iconv-dir=DIR are there to help you specify them. When asking for support, always mention these directives, and whether there are other versions of those libraries installed on your system somewhere. Naturally, provide all the version numbers.