On Unix, run configure with the --enable-xslt --with-xslt-sablot options. The Sablotron library should be installed somewhere your compiler can find it.
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.
Be sure your Sablotron library is linked to -lstdc++ as otherwise your configure will fail, or PHP will fail to run or load.
Note: Note to Win32 Users
In order for this extension to work, there are DLL files that must be available to the Windows system PATH. For information on how to do this, see the FAQ entitled "How do I add my PHP directory to the PATH on Windows". Although copying DLL files from the PHP folder into the Windows system directory also works (because the system directory is by default in the system's PATH), this is not recommended. This extension requires the following files to be in the PATH: sablot.dll, expat.dll, and iconv.dll
For PHP <= 4.2.0, the file iconv.dll is not required.