I'm currently fixing a little problem with a program I use at work. I had to add a call to exsltRegisterAll() from libexslt (the library that's bolted onto libxslt that implements the EXSLT XSLT extension functions) to fix the problem.

Now pkgconfig has configurations for libxml and libxslt, but some bright spark forgot to allow programmers to add support for detecting/linking-in libexslt. There's no M4 script included anywhere in libxslt either to do the job. Necessity is the mother of invention, I guess.